The Top Mac
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
Audacity - Audacity is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.
Alarm Clock - A native alarm clock for Mac OS X. It runs out of the way, up in your system tray by your clock. You can set multiple alarms to go off whenever you'd like, and you can wake up to any song in your iTunes music library. It can also wake your mac from sleep, so no need to keep your mac up and running all the time.
Cyberduck - an open source FTP and SFTP (SSH Secure File Transfer) browser licenced under the GPL with an easy to use interface, integration with external editors and support for many Mac OS X system technologies such as Spotlight, Bonjour, the Keychain and AppleScript.
djay - the groundbreaking new software for Mac OS X. It takes full advantage of Apple's latest audio and graphics technologies.  With djay you can control and mix your whole digital music collection with two realistic virtual, interactive turntables...
GrandPerspective - a small utility application for Mac OS X that graphically shows the disk usage within a file system. It can help you to manage your disk, as you can easily spot which files and folders take up the most space. It uses a so called tree map for visualisation. Each file is shown as a rectangle with an area proportional to the file's size.
HandBrake - a GPL’d multiplatform, multithreaded DVD to MPEG-4 ripper/converter. HandBrake was originally available on the BeOS, but now has been ported over to MacOS X and to GNU/Linux.
iSquint - an iPod video conversion app for Mac OS X.
It's many times faster than QuickTime Pro, works with almost all popular video formats, and it's infinitely free-er.
It's also really easy.
Just drag in your file, and click Start.
Paparazzi! - Paparazzi! is a small utility for Mac OS X that makes screenshots of webpages.  More importantly, it can capture long webpages that you would normally have to stitch together.
Quıcĸsıɩⅴεʀ - A unified, extensible interface for working with
applications, contacts, music, and other data. Quicksilver’s greatest strength, however, is not search. Any item you are able to find, drag, or otherwise pull into its universe is endowed with many potential uses.
Senuti - a simple application for transfering songs from your iPod back to your computer. Senuti is the only alternative that will give you the power and convenience that you need, wrapped in an interface that is extremely easy to use, at a cost that you can't complain about.
Transmission - Transmission is a free, lightweight BitTorrent client. It features a simple, intuitive interface on top on an efficient, cross-platform back-end.
Transmission is open source (MIT license) and runs on Mac OS X (Cocoa interface), Linux/NetBSD/FreeBSD/OpenBSD (GTK+ interface) and BeOS (native interface).
VLC - a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols.