system (8)
Butler
http://www.petermaurer.de/
This is probably the most awesome app ever conceived. Contains most of the functionality of LaunchBar and Quicksilver + several other apps too, all with a tiny memory footprint.
ClamAV
http://www.markallan.co.uk/clamXav/
If you are worried about viruses, here is a free scanner.
iTerm
http://iterm.sourceforge.net/
If you use the Terminal a lot for the UNIX backend of OS X, this thing is definitely worth a try. Can even have tabbed Terminals. UB.
MenuMeters
http://www.ragingmenace.com/
If you like to keep an eye on CPU use, bandwidth, mem usage, this will give you the readouts. Essential on every working person's Mac.
OnyX
http://www.titanium.free.fr/pgs/english.html
Excellent utility to optimize parts of OSX and turn on and off various things such as the viewing of hidden files. UB.
OSXvnc
http://www.redstonesoftware.com/vnc.html
Incredibly useful if you want to access your computer from away from home.UB
SysStat Nano
http://www.islayer.com/viewWidget.php?id=21
This is a Dashboard widget which can show almost every stat about your computer at the click of a button.
