Condoms and dental floss
Just a quickie, sorry if it's been said before. Two items I like to pack, for more than the obvious use. Condoms are great if you need to waterproof something fast. This may be a sudden downpour or an imminent water crossing. Air filters, black boxes and tailpipes for example. Personal electrical stuff like cameras, ipods and notebooks too. The possibilities are endless and you'd be surprised how quick you can get them on when you need to!
Dental floss is ace if you need to affect a temporary repair. You'll be surprised how strong this stuff is, it's easy to work with and can be used to secure many things. Both items are cheap, easy to pack/store away and can indeed be a lifesaver, please forgive the unintentional ambiguity!