Really? That could be someones only paycheck to feed their family on, or an elderly person's SSA money. If it was your money, would you want someone to just take it?! Do the right thing. This is theft even if you could "get away with it"!
No you can't be arrested. Seriously that could have been anyone's money so who's to say it's not your money. Plus no one can prove who's money that was to begin with so either you can spend it or someone else can lie and it say it's theirs.
It depends on where you found it, how you found it, how much it is and whose money it is. If you are in Walmart and you did not see anyone drop it, it's a small amount like $5 and it is just laying there with no way of knowing who it belongs to then I do not believe you will get in trouble. If it is a large sum of money I would turn it into customer service as someone is probably pretty stressed out about losing their money.