Andre Torrez was inspired to build a Twitter add-on service that allows you to track what happens to the accounts you report for spamming. Later, Spam! remembers the spam reports you've made and keeps track of whether Twitter has deactivated those accounts, giving you a little running tally of how many spammers' accounts you've helped to nuke.
In my experience there isn't much of a spam problem on Twitter. Yes, it's annoying to mention something about your iPad and have a spam bot or two tell you how you can get a free one just by "clicking this URL," but I feel like that happens once or twice a month at most.
I normally just mark the thing as spam and move on. But the last time it happened I clicked over to see the account's timeline and saw they had been at it for quite some time. Even tweeting innocuous tweets in between the mention spam which I guessed was to throw off Twitter's own spam algorithms…
So I built laterspam.org because I thought people might get a little satisfaction out of marking something as spam and knowing Twitter did something about it.