Best way to clean up my hard drive?

When I had a PC, I use to go into disk clean up or whatever to delete all those old unused files off my computer. I've had my Imac for about 2 years and I would like to know a program that will help clean up my hard drive. I downloaded something onto my computer and deleted it, but it is still showing that the space is being used, so I'm wondering if something is hiding somewhere. What kind of program would be best to use here? Thanks!

http://www.thexlab.com/faqs/freeingspace.html

You can use OnyX to delete caches, logs, etc...

Also, the Mac "fixes" itself while on, but not in use and not while sleeping. There are scripted maintenance programs that run on a daily, weekly, and monthly basis.
A program you can use if not left on a lot.

MacJanitor

http://personalpages.tds.net/~brian_hill/macjanitor.html