BetterTouchTool - An incredibly useful tool that helps me get the most out of my Magic Mouse/Trackpad. I currently have it configured for window-snapping (like in Windows 7), and pinch to minimize/maximize my windows.
- A very useful IM client, the one I use the most.
Twitterrific - My preferred Twitter client. I like it because of its clean, sleek, minimalistic interface; as well as the choice of colours. Helpful navigation buttons are also there to get me where I need to be quickly- whether it be messages, profile etc. 
Last.fm - Scrobbles stuff I listen to; I only use it to scrobble music I’ve played on my iPhone or iPad. 
ManyCam - Very useful webcam control. I can set this as 1 input to anything that uses video or webcam. I can then set the webcam source to a picture, video, etc. I use this a fair amount.
Dropbox - For syncing files on the cloud. Acts as a 2nd USB drive for me, useful for transferring and sharing files.
Bowtie - Clean, out of the way, minimalistic interface. I use this for scrobbling things I listen to in iTunes to Last.fm.
Alfred - Search for files quickly; much better and faster to use than Spotlight search. Also more minimalistic, key commands very useful.
CloudApp - I use this to upload screenshots, pictures etc. Very clean, sleek web interface. Auto-copies the picture link to my clipboard which is useful.
DisplayPad - A great app. I can use this to extend my display to my iPad; using it as a 2nd, 3rd etc monitor. Very low latency- even though it uses WiFi!
WiFi2HiFi - Brilliant streaming app. Sometimes can be out of time, but can be useful in a lot of situations!
LogMeIn - Remote Desktop app. Free, which is great. Works well with mac- speedy and responsive. 
Facebook Notifications - I can see what’s going on on Facebook without opening my browser!
iChat - Set chat status, etc. Quite useful.
Bluetooth - I mainly use this just to connect my Magic Mouse, and it tells me whether the battery needs replacing.
AirPort - Default WiFi on the Mac, shows what base station I’m connected to + signal strength etc.
Volume control - I normally use this with the ‘Option’ key, so I can quickly change audio inputs/outputs.
Battery Indicator - I just stuck with the Apple indicator, with battery percentage.
Character/Keyboard viewer - helpful when entering characters with accents on them etc.
Time - Plain and simple- the time, with seconds and the day. 
Fast User Switching - I don’t use this to fast user switch, but just to lock my Mac.
Spotlight Search - searching for things on my Mac. It still has its uses (ie a calculator), but I normally prefer to use Alfred instead (see above).
(From Jamie Goodliffe)