Firebug

getfirebug.com
Mac OSX, Windows, Linux
I consider this Firefox addon a MUST for anyone involved in website development. With it you can view and edit the source and CSS code for sites your building IN REAL TIME while viewing the pages in firefox. Use the built-in inspector to drill through your own code or to see what other sites are empoying. Try out the debugging console for javascript or PHP. Validate your code. Get that placement just right with the DOM editor... and so much more.
Google Chrome

google.com/chrome
Windows
Google's newly launch web browser Chrome has got to be the fastest one out there for Windows. You can tell they put a lot of thought into it's clean design (what they are calling "one box") and interesting features (favorite thumnails, application shortcuts, crash control, etc).
Deploy Studio

deploystudio.com
Mac OSX, Windows, Linux
Made by Apple engineers in France (though not offically sanctioned by Apple) Deploy Studio is an excellent replacemnet for the now end-of-life NetRestore. With Deploy Studio you can create a master image of any Mac or PC and clone that image through your network to any number of other computers. BUT WAIT, there is so much more. The program is designed so you may easily setup pre and post imaging events in a workflow. So as an example, you could setup a workflow for your school's video production lab that went like so:
  1. Create two partions on the drive one for OSX one for Windows.
  2. Blast my mac and PC master images to those new partitions.
  3. Install my video editing software.
  4. License said video editing software.
  5. Run some more post install scripts.
  6. Setup the proper network settings, and bind this computer to my directory.
  7. Etc, etc.
Omni Dazzle

www.omnigroup.com
Mac OSX
Adds all kinds of fun functions to your mouse when doing presentations via a keyboard or mouse hot key.
MacLoc

ourapples.com
Mac OSX
This simple application activates the Fast User Switching feature built-in to Mac OS X. After clicking on MacLoc, it will appear that you've logged out, however upon entering your username and password, all open documents and applications will be right where you left them. MacLoc is the perfect solution when you need to step out of the room for a moment.
EverSave

www.tool-forcesw.com
Mac OSX
EverSave is a clever tool which allows you to save all of your documents - if possible - in a specific time interval or by changing the frontmost application. This means no more data loss if applications crash.
ScreenToaster.com

screentoaster.com
Web-Based
There are quite a few free online screencasting sites out there. So far screentoaster.com takes the cake.
It is as simple as clicking record, run through your tutorial on your computer, and click stop. The recorded video can be exported to a number of formats to share with others and post online. What really sets ScreenToaster apart is it's ability to use practically and audio or video input while recording. Plus you may do simple edits to the video after recording right through their interface.

NameChanger

mrrsoftware.com
Mac OSX
NameChanger is designed for the sole purpose of renaming a list of files.
Bonus, you get to See how the names will change as you type.
Cyberduck

cyberduck.ch
Mac OSX
An easy to use FTP client for OSX. Even comes with a widget interface and example applescripts showing how to automate FTP transfers.
Angry IP Scanner

www.angryziber.com
Mac OSX, Windows, Linux
Angry IP scanner is a very fast IP address and port scanner. It can scan IP addresses in any range as well as any their ports. Useful in analyzing firewall rules and security concerns. It is cross-platform and lightweight.
NeoOffice

neooffice.org
Mac OSX
What could be better than the free feature-rich OpenOffice program suite?  How about an OSX native  version (no need for X11)!  In addition to word processing, spreadsheets, and presentations with NeoOffice you can create drawing documents, HTML pages, business cards, and master templates.
Google Earth

earth.google.com
Mac OSX, Windows, Linux
Well if you have yet to try out Google Earth you are missing out. This program is an amazing geographic vistualizer and mapping tool. Check out Google's education site for suggestions on how it can be incorporated into the classroom.
Firefox

mozilla.com
Mac OSX, Windows, Linux
Probably the most widely recognized open source program, Firefox is a very secure and versatile web browser. Some of my favorite classroom add-ons to Firefox include: adblock menu editor public fox. And if you ever need a hand with something there is a great support community at mozilla.com.
Phpicalendar

phpicalendar.net
Web-Based
So I have my school's event calendar in iCal and want to post it to the district's web site. Well PHPiCalendar was created for just that purpose.  The program is simply a folder full of PHP pages that will read an .ics file and make it into a web site, with all the navigation built in.  PHP iCalendar is compatible with a large number of calendaring programs other than iCal.
iRedLight

www.filewell.com
Mac OSX
Greatly extends the use for that tiny Apple remote that came with newer Macs. It allows the remote to control many system actions and programs including PowerPoint, NeoOffice, and QuickTime. Comes in very handy for presentations in and out of the classroom. The remote buttons can also be mapped to applescripts (the possibilities are endless).
Gawker

gawker.sourceforge.net
Mac OSX
Makes time-lapse photography/movie making extremely easy using the built-in iSight on the new macs, or any attached camera. Just input the interval between frames, and what movie output you would like. Interesting also is it's ability to attach to remote cameras attached to other macs through bonjour.
Thunderbird

www.mozillamessaging.com
Mac OSX, Windows, Linux
This open source email client is maintained by the same community that develops Firefox (Mozzilla).  It is a very clean and easy to use email program.  Basically all the best parts of Outlook, without the clutter.
iConcertCal

iconcertcal.com
Mac OSX, Windows
Being a big concert goer I was so happy to stumble onto iConcertCal. It is an iTunes visualizer that will show you upcoming concerts and CD releases for the bands in your iTunes library. With nice rollover effects and direct links to buy tickets I think I can finally stop pouring over the early warning section of The Reader.
MAMP and WAMP


Mac OSX
The ultimate web programming and design test tube.  Both WAMP and MAMP are self-contained environments running Apache, MySql, and PHP.  Meaning you can develop "traditional" websites and/or complex CMS web portals from the comfort of your personal computer. No knowledge of setting up web servers is required. WAMP is for Windows, and MAMP for Mac folk.
SpiceWorks

www.spiceworks.com
Windows
Is a network monitoring and inventory type software.  It can track and give all kinds of details about nodes on your network (PCs, Switches, Routers, Printers, etc). Unfortunately it must be run from Windows. But it can report on OSX, Linux, and Unix stations just fine.
Wink

www.debugmode.com
Windows, Linux
One piece of freeware I really wish they made for Mac. Wink is a Tutorial and Presentation creation software, primarily aimed at creating tutorials on how to use software (like a tutor for MS-Word/Excel etc).  Since you can save in flash format Wink makes it extremely simple to publish your tutorials to the web.
Transmission

www.transmissionbt.com
Mac OSX
My favorite Bittorrent Client. Extremly Lightweight with just the right amount of features.
iClockr

www.kedisoft.com
Mac OSX
iClockr is focused to show you a simple way to track your time. Great for calculating billable hours of freelance work.
Cronnix

serverkompetenz.net
Mac OSX
A GUI frontend for creating cron jobs.
MacTracker

mactracker.ca
Mac OSX, Windows
A helpful program for people in charge of maintaining or repairing Apple products.  It is basically a database of all the specifications for every Apple product ever made. Constantly updated with new products by the developer.