Only adding those that haven't been mentioned (I think - I may have doubled up on some of them).
ClassicFTP - used so I can describe FTP operations to people using Windows. When I am using FTP I tend to use the FireFTP plugin for Firefox.
DbVisualizer - a great way of accessing the various databases I have to use
Druid - another nice database tool
FinkCommander - for those *nix tools that the Mac doesn't have, or that I want a newer version of
Flip4Mac - for playing WMV files
Gimp - for graphics manipulation
JungleDisk - for remote access to my S3 space as though it is a local hard drive
NeoOffice - for word processing / spreadsheeting etc. I should probably revert back to Open Office.
OSXvnc - remote access to other computers
Parallels - allows me to run other operating systems on my computer
Quicksilver - all sorts of goodies
Skype - VOIP
Eclipse - IDE
hudson - continuous integration tool
hudsonTracker - nice tool to monitor hudson
iSquint - convert my movies into iPod format
sshfs - easy way of accessing some remote computers as though they are
And in my games folder (for when the kids come around - honestly - really truly)
I've tried QuickSilver on my still new Mac for a while, but then discovered that the built-in Spotlight can do almost (?) anything that QuickSilver does. Does QuickSilver have extras that make it worthwile to use it instead of Spotlight? I'd rather not install and have another program running if it's not essential.
I like PixelMator, it's a graphics editing program, ofcourse not as large and powerful as Photoshop, but not that expensive and works really well and has a lot of functionality.
For Java and Scala development I use NetBeans.
I use Firefox for web browsing.
I use VirtualBox (instead of Parallels) for running Ubuntu and Windows XP in a virtualized environment. VirtualBox is free and works really well.
I use OpenOffice for documents and spreadsheets.
I also have VLC Media Player installed, it plays more formats than the QuickTime can handle.