It depends on my mood. Generally not coppers (1p and 2p coins), the smaller silvers (5p and 10p coins) are a hit or a miss, and I'll generally pick up anything higher than that (20p, 50p, £1, £2 coins and any notes). Oh, and obviously, if I found the money in a wallet or something that had identification in it, I'd either hand it in to the authorities or track down the owner and return it. Unmarked currency on the streets though? Finder's keepers.
The most I've ever found in one go was a £20 note, though it was years ago and I can't remember details such as what I spent it on or anything, but I do remember it was a windy day and it was pinned to a hedge by the wind as I walked past.
I also remember one time when, for some reason, I was about 10p short for my bus ticket home, and managed to make up the money solely in coppers I found on the ground on my walk to the bus station. That was a fortunate occurence.