New Version!
CCleaner is a freeware system optimization and privacy tool. It removes unused files from your system – allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. But the best part is that it’s fast (normally taking less than a second to run) and contains NO Spyware or Adware! 
What’s new in 2.0.2.525 :
v2.02.525
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- Application is now 100% Unicode, with support added for non-Unicode OSs.
- Fixed bug where some IE temp files were not being securely deleted.
- %LocalAppData% and %CommonAppData% variable support added to INI files
allowing “Application Data” references to work on non-English versions
of Windows.
- Fixed bug where “Run (in Stat Menu)” was being cleaned when deselected.
- Added Gutmann (35 pass) secure deletion.
- Fixed minor bugs with 98 and NT4.
- Fixed bug with update checker.
- Added “portable.dat” file to the portable version of CCleaner, which
forces it to save the settings to an INI file.
- Fixed bug in cookie scanning that could cause an exception.
- Fixed minor UI bugs relating to control and window focus.
- Improved exception handling with more useful information.
- Minor bugs fixes and tweaks.
Official Website
I never used this software but a lot of my friends use it and talk me about it.