Spoiling your vote is the act of casting a ballot marked or mangled in such a way that it cannot be counted as a vote for any candidate. This is seen as sending a message of discontent with the current viable choices in political parties, as opposed to the message presumably sent by not voting at all, a message of apathy.
Spoiling your vote seems to be most popular as a movement in the UK, where it is trying hard to become a grassroots movement. The UK is ripe for this sort of movement, as they currently use paper ballots that are easy to spoil either by marking a large X over the face of the ballot or by ripping them in two, and their voter turnout has plummeted to a disgraceful 65.1%. (The US, incidentally, had 56.8% turnout in the 2008 presidential election).
As far as I can find, there is nowhere in the world where spoiling your vote carries any sort of official or political weight. Spoiled votes are generally reported and appear as a publicly visible statistic, but no action is taken based on their appearance.