Free Apps

• QuickSilver – the definitive time saving app getting things done without doing them
• Growl – displays messages directly on the screen in a little transparent window
• Adium – Instant Message app that works with just about every protocol out there today
• Chicken of the VNC – excellent VNC client
• iStumbler – for finding WiFi hotspots
• NeoOffice – OS X implementation of OpenOffice.org (a Microsoft Office replacement)
• xCode – Apple’s IDE, this gives you C compilers and other tools you will need for compiling things like Ruby gems
• SvnX – Subversion Client (if you don’t want to use the command line)
• iSquint – converts just about any video format into iTunes, Apple TV, and iPod compliant video
• MenuMeters – puts a set of graphs in the menu bar that show you what your CPU, Memory, Networks connections, Disks are doing
• Perian – added codecs for playing back non quicktime compliant audio and video files

Not Free

• TextMate – the only editor for the Mac
• Transmit – FTP/SFTP client
• Omnigraffle – Visio replacement for creating diagrams
• World of Warcraft – I think this one goes without saying

That is all I can think of right now, as more come to me, I will add them.