Piriform

For computer maintenance, the freeware offered by Piriform would be great to have as portable apps.

http://www.piriform.com/

CCleaner: clean out browsing history and it even finds and fixes registry errors
Defraggler: A much better defragmenting tool then the one offered by Windows
Recuva: a way to recover files deleted on accident
Speccy: See specs and temperatures of the components on the machine