You can sell you background of your twitter account to advertisers thanks to Twittad.
You choose the price you want for your background and when a client is meeting your expectations, you get a campaign. It's quite easy to make money.
At the moment I am testing one of their campaign on my Twitter and I don't think it's really annoying. It's only the background, this does'nt alterate my twitter activity and don't pollute my followers.
What do you think?