As far as I’m concerned, no Mac is complete without these great freebies..
Cyberduck
Great SFTP/FTP rich-client.
Colloquy
An intuitive, native IRC client
VLC
Dirt simple media player capable of handling things that QuickTime can’t.
Handbrake
All-in-one DVD ripping and encoding utility. Provides plenty of features for most of us without be overly complicated.
Skype
Easy-to-use VOIP client. Works with MacBook (Pro)/PowerBook microphones out-of-the-box, and provide free calls to other Skype users.
CrossOver Mac Beta
Every OS X user I know must operate in a Windows environment on a regular basis. The recent release of Parallels for Intel-based macs (basically VMWare workstation-style guest-OS host virtualization) has made life one tick easier by allowing us to run Windows on our Core Duo systems, and the new CrossOver Beta for Intel Macs provides a great complementary technology. CrossOver Mac, by CodeWeavers of wine fame, allows you to run some Windows applications directly on your Intel Mac, without having to host a full-fledged copy of Windows by implementing the applications APIs dependencies on the Mac. The beta is free.
QuickSilver
Provides Spotlight-like capabilities, but faster and way, way better. The extremely brisk find-and-launch concept will be new for many users, but will be hard to live without once understood.