CCleaner for U3 is a freeware system optimization, privacy and cleaning tool that can be run from your U3 capable removable drive. 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. Additionally it contains a fully featured registry cleaner. But the best part is that it's fast.

CCLeaner for U3 cleans the following:

•  Internet Explorer
  Temporary files, URL history, cookies, Autocomplete form history, index.dat.
• Firefox
  Temporary files, URL history, cookies, download history.
• Opera
  Temporary files, URL history, cookies.
• Windows
  Recycle Bin, Recent Documents, Temporary files and Log files.
• Registry cleaner
  Advanced features to remove unused and old entries, including File Extensions, ActiveX Controls, ClassIDs, ProgIDs, Uninstallers, Shared DLLs, Fonts, Help Files, Application Paths, Icons, Invalid Shortcuts and more... also comes with a comprehensive backup feature.
• Third-party applications
  Removes temp files and recent file lists (MRUs) from many apps including Media Player, eMule, Kazaa, Google Toolbar, Netscape, MS Office, Nero, Adobe Acrobat, WinRAR, WinAce, WinZip and many more...

Update 29/06/2009
CCleaner is now official available from developers website, i will no longer provide my own package, http://www.ccleaner.com/download/builds