Steve's list of essential mac freeware
all of this stuff definitely runs on Snow Leopard (10.7) and *should* run on Lion (10.8). Let me know if you find something that doesn't!

system tools and utilities

essentials:
• Growl (popup notifications from apps):
• Google Notifier (Google mail and calendar alerts)
• DropCopy (local network file copy to other mac users) - I still like this, but haven't had another Lion user to test AirDrop with...
• Grand Perspective (where *did* all my disk space go?)
• AppCleaner (application remover)
• DropBox (Cloud storage that syncs files on your computer)

geeky stuff:
• xMod (tweak hidden system prefs)
• Warp (move between spaces with mouse)
• iStumbler (wireless discovery tool)
• iTerm (a nice terminal replacement)
• iTerm2 (a better nice terminal replacement)
• Name Mangler (bulk file renamer)

quicklook plugins:
• QuickLook Plugins (these may not all work with Lion)

communication

essential:
• Adium (multiple IM service client - similar to iChat)
• Skype (internet calling, requires free account)

geeky stuff:
• Colloquy (nice IRC client)

internet

essential:
• Choose Wisely (web browser chooser when you click on links; make it your default browser via Safari Preferences):
• Google Chrome (web browser):
  • Chrome Webstore (apps and extensions)
• Firefox (web browser)
  • Firefox Add-ons
• Dropbox (online file storage and syncing, 2G free - using this link gets us both more space!)
• Google Earth

geeky stuff:
• Transmission (light-weight, mulitplatform BitTorrent client)

graphics

• GIMP (graphics program a la Photoshop)
• Seashore (mac-native alternative to the GIMP)
• Inkscape (open source vector graphics editor)
• Skitch (quick image editing and sharing)

office

• Google Docs (free web browser/cloud-based office suite)
• LibreOffice (an off-shoot of OpenOffice - use this instead of OO!)

development (all geeky)

programming:
• MacVim (port of the Vim editor)
• Komodo Edit (multi-language editor)

web development (browser addons):
• Firebug (very useful web development tool)
• ColorZilla (color picker/tool)

multimedia

• VLC (multi-format player)
• Plex (media center)
• Handbrake (DVD to MPEG-4 converter)
• MactheRipper (another DVD extractor)
• Burn (audio/video/data CD/DVD extractor and burner)
• Audacity (audio editor)
• Picasa (Photo editing and organising software from Google - an alternative to iPhoto)

geeky:
• Soundflower (Inter-application Audio Routing Utility)

games

• Quinn (Tetris)
• Stella (Atari 2600 VCS Emulator)
• VICE (Commodore 8-bit computer emulator)

toys

• MacSaber (turn your mac laptop into a lightsaber)
• SeisMac (monitor seismic activity on your laptop)

screensavers

• Soundstream (uses the microphone as input)
• Desktoy (perpetual motion)
• Fenetres Volantes (flying windows)