I think TortoiseSVN Portable would be a great addiction to portable apps. Ok before all you nay-sayers come in. Yes I know of SubCommander and RapidSVN but neither have a very good set or features, they aren't very stable and they are not easy to use! and Yes I know it's a shell extension! but I really do think it would be perfectly eligible to be a Portable App. First off it is Opensource and OSI compliant, Secondly TortoiseSVN *really really* simplifies SVN, just about anyone can use it.
I'm sure there's more too it than just registering/unregistering the shell extension, but this would totally make my day if this was made into a reality! I promise not to get my hopes up.