Tests IE5.5, 6, 7 & 8 beta. Just downloaded it and had a quick look and it seems to work as intended. Probably too early for there to be much feedback about how faithfully it simulates the different versions.
The idea is great, but it appears to have a lot of bugs and problems. I'm just reading the forum for it at the moment:
Still, it could also save a lot of time and actually looks quite nice! Here's to hoping he succeeds in his attempts to fix it
I couldn't see any
It does seems to have some other useful features, similar to Firebug.