Level
I was about to put up some shelves yesterday when I realised that I couldn't find either of our spirit levels. Don't ask me what became of one of them, but the other turned out to be on the allotment with Mr Bsag (vertical carrots, perhaps? I don't know, I'm as baffled as you are.)
Anyway, I wanted to get on with the job and suddenly realised that there are many spirit level apps available for the iPhone. Bingo, job done. You can see the results below (one of the shelves). I'm not sure I would use it to build a house, but it was accurate enough for a couple of shelves.
