BetterTouchTool - Lets you define tons of gestures for your Macbook’s Trackpad, your MagicMouse and your MagicTrackpad. In addition to that it brings lots of new stuff to MacOS like Windows 7 like window snapping, window switchers etc…
Last.fm Scrobbler - To add the music you play to your profile.
Caffeine - Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers.
Wally - Wallpaper changer, using multiple sources like files, folders, FTP remote folders, Flickr, Yahoo!, Panoramio, Pikeo, Ipernity, Photobucket, Buzznet, Picasa, Smugmug, Bing, Google, Vladstudio and deviantART images.
Butler  - To ease all those routine tasks you do every day: controlling iTunes, opening programs and documents, switching users, searching for stuff on the web, and more.
iMac (customized by system icon) - Open programs and documents.
The rest: Preference pane, Google, Bluetooth, AirPort, iStat Menus (Network, Disk Activity, Memory, and CPU), Time Machine, Date & Time, かわせみ, Japanese input method, and Spotlight.
(From )