Wikipedia has an article on the algorithm, perhaps you could convert the pseudo-code to Processing code.

Finding the pseudo-code in the article a bit cryptic, I read the JavaScript implementation and took ideas and formulae from it. I rewrote it in a more Java spirit, with a Processing display, of course...
The result can be found at http://bazaar.launchpad.net/~philho/+junk/Processing/files/head:/_SmallPrograms/RansacAlgorithm/ (several tabs)
I plan to add some little improvements, like seeing if a hand-given line has a better score and if running the algorithm some more steps can also improve the result.

I added the features described above.

- Left click add a point to those already generated;
- Right click launches the algorithm;
- + increase the speed;
- - decreases it;
- * adds 5 iterations after the algorithm has ended (more tries);
- / enters the user mode: the user can click on two points to see if the lines between them has a better score. If so, the best score is updated. User can choose another pair for another try.
- c clears the current set of points and start over.