Mac
Adium
Multi-protocol chat client.
Bwana
Read man pages in your browser.
Chicken of the VNC
Mac OS X VNC client.
http://sourceforge.net/projects/cotvnc/
Flip4Mac
WMV support for the Mac.
Google Notifier
Quick access to your Google mail and calendars.
http://toolbar.google.com/gmail-helper/notifier_mac.html
Growl
Nice notifications - integrates with lots of other apps.
iScrobbler
Mac iTunes support for last.fm.
http://iscrobbler.sourceforge.net/
LastFM
Mac player application for last.fm
MySQL Tools
User interfaces for browsing and administering SQL databases.
http://dev.mysql.com/downloads/gui-tools/5.0.html
NetNewsWire
RSS feed aggregator.
http://www.newsgator.com/Individuals/NetNewsWire/
OmniGraffle
Great diagram drawing application.
http://www.omnigroup.com/applications/omnigraffle/
OmniOutliner
Hierarchical lists galore.
http://www.omnigroup.com/applications/omnioutliner/
Parallels
Run Windows on the Mac.
http://www.parallels.com/en/products/desktop/
SharePoints
Nice little UI for configuring AppleShare and SMB file sharing.
http://hornware.com/sharepoints/
SubEthaEdit
Snappy collaborative text editing.
http://www.codingmonkeys.de/subethaedit/
The Filter
Music recommendation service.
Twitterific
Desktop Twitter client.
http://www.apple.com/downloads/macosx/email_chat/twitterrific.html
xCHM
A Mac CHM reader.
http://xchm.sourceforge.net/ http://www.macupdate.com/info.php/id/13750
Windows
ObjectDock
A Mac style dock on the PC, yay!
http://www.stardock.com/products/objectdock/
Cross Platform
DOS Box
DOS emulator.
http://dosbox.sourceforge.net/
Doxygen
Source code documentation generator.
Eclipse
Cross-platform IDE.
Flock
Firefox derivative web browser, with lots of nice support for other services.
Freemind
Mind mapping software.
http://freemind.sourceforge.net/
Perforce
The king of source control.
Skype
Another chat / voice-over-IP client.
Synergy
Share your mouse, keyboard and clipboard between multiple machines. It just works!
http://synergy2.sourceforge.net/
Thunderbird
Cross platform email client. Doesn’t suck too hard.
http://www.mozilla.com/thunderbird/
Visual Paradigm
Nice cross platform UML modelling application.
http://www.visual-paradigm.com/
Yahoo! Widget Engine
Cross platform desktop widgets.