One clue - it's a very small car. That's it. The colour should be a dead giveaway.
EDIT: If you're just joining us, here's a rundown of where we are so far:
- Minis, Peel P50s, etc etc etc - way too big
- Barbie car - too big
- Hot Wheels - too small
EDIT 2: I'm going to bed soon, and it wouldn't be fair to leave you all hanging, would it? So here's another picture...
EDIT 3: We have a winner! http://oppositelock.jalopnik.com/lego-simpsons-...