Hands down the quickest way to make a lot of money (well, without breaking the law). Lots of students have genuinely made £100s from this technique. It's completely legal, risk free, tax free, and anyone over 18 in the UK can do it (Not in UK? Skip to no. 2).
It works by taking advantage of free bets regularly offered by betting sites through 'matching' them at a betting exchange. Matched betting eliminates the risk (you are betting both for and against a certain outcome).
This leaves you being able to squeeze out the free bet, which can be as much as £100. Multiply this by how many betting sites there are and you can quite easily come away with a profit of a few hundred pounds.
