Cool Apps
Utilities
MovieSync allows you synchronize audio and video in QuickTime movies. You can also synchronize other QuickTime readable media, such as AVIs, MPEG-4, and, with appropriate codecs, even WMVs.
VirtueDesktops is a virtual desktop manager for Mac OS X. It offers features, eye candy and configurable options that no other desktop manager on the mac has added yet, and is under active development. It is based upon DesktopManager, which is a lighter, less feature-packed virtual desktop application.
MenuMeters is a set of CPU, memory, disk, and network monitoring tools for MacOS X. Everything is displayed in the menu bar. MenuMeters uses Apple's private API for menu items, so you need to install Menu Extra Enabler to allow MenuMeters to gain access to that API.
QuickSilver, a unified, extensible interface for working with applications, contacts, music, and other data. I use it to get fast keyboard access to things like applications or preference panes, and also because there are various modules that do cool things, like let you bind keyboard shortcuts to controlling itunes, or binding keyboard shortcuts for arbitrary actions on arbitrary data. Particularly useful is binding hotkeys to pasting specific chunks of oft-repeated text (useful for anybody who has to do any sort of tech support, heh).
Plugins / Enhancements
Chax is a collection of minor modifications and additions that make using Apple's iChat more enjoyable. Notable features: tabbed chat windows, preference to not pause AV sessions during file transfer.