My first choice here is CCleaner [1], a product that started off as pretty basic but has continued to improve to the point where it is now very close to the best in its class regardless of price. It's very effective in freeing up disk space by removing unused and temporary files, cookies, Recycle bin entries, Windows hot-fix files, old prefetch data, history and cache files in many applications such as Internet Explorer and Firefox and much more. Additionally it will check the Windows Registry for invalid entries.
There are some minuses: first the installation settings are quite aggressive so do check them carefully. Second the product installation, by default, includes the installation of the Yahoo Toolbar so if you don't want it, make sure you uncheck that option. Finally I wouldn't trust this product to do your Registry cleaning; that's a job for a specialist utility. Similarly if your aim is to remove all your surfing traces rather than just to clean up your disk I'd be inclined to use a specialist cache cleaner.
However if simply freeing up disk space is your objective the CCleaner is hard to beat.
A good companion to CCleaner is Empty Temp Folders [2]. It's not as complete a product as CCleaner and it's been a while since it was last updated however I find it always manages to clean more temporary file than CCleaner. That's why I suggest you use both.
Another cleaner with a solid following is Steven Gould's CleanUP! [3]. It's a compact, well designed and very well maintained and is a real alternative to CCleaner. I've had a couple of instances of it causing problems on my test PC but this may be specific to my setup. Certainly the feedback I get from users has been uniformly excellent.
In a related category is PC Decrapifier [4]. Its job is to remove unwanted trial versions of programs and services that come preinstalled on new PCs. You can remove all this stuff manually but PC Decrapifier does this automatically. It can't remove every bit of junk from every computer but the list of products [5] it will remove is quite impressive.
[1] CCleaner
Website: http://www.ccleaner.com/
Download link: http://www.ccleaner.com/download
Author: Piriform Ltd.
Current version: 2.0.7.575
Version date: April 28, 2008
License: Freeware
Download Filesize: 2,830KB
Operating Systems Supported: Win9x/Me/NT4/2000/2003/XP/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: Yes
Non-English languages supported: Albanian, Arabic, Bosanski, Bulgarian, Català, Cesky, Chinese (Simplified), Chinese (Traditional), Dansk, Deutsch, Español, Français, Galego, Greek, Hebrew, Italiano, Japanese, Korean, Lietuviu, Macedonian, Magyar, Nederlands, Norwegian, Polski, Português, Português (do Brasil), Romana, Russian, Serbian Cyrillic, Serbian Latin, Slovensky, Suomi, Svenska, Türkçe
Other relevant information: None
[2] Empty Temp Folders
Website: http://www.danish-shareware.dk/soft/emptemp/
Download link: http://www.majorgeeks.com/download1575.html
Author: Finn Ekberg Christiansen
Current version: 2.8.3
Version date: August 2001
License: Freeware
Download Filesize: 651KB
Operating Systems Supported: Win9x/Me/NT4/2000/2003/XP/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: None.
Other relevant information: None
[3] CleanUp!
Website: http://cleanup.stevengould.org/
Download link: http://www.stevengould.org/software/cleanup/download.html
Author: Steven R. Gould
Current version: 4.5.2
Version date: June 26, 2006
License: Freeware
Download Filesize: 332KB
Operating Systems Supported: Win9x/Me/NT4/2000/2003/XP/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: None
Other relevant information: None
[4] PC Decrapifier
Website: http://www.pcdecrapifier.com/home
Download link: http://www.pcdecrapifier.com/download
Author: Jason York
Current version: 1.9.1
Version date: April 30, 2008
License: Free for Personal use. Commercial License available.
Download Filesize: 672KB
Operating Systems Supported: XP/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: None
Other relevant information: