Litmusapp may be nicer and have summaries, but it's hard to beat browsershot's simplicity, breadth, and free-ness. Though I have yet to see any provide decent testing for anything dynamic (videos, perhaps?).
browsershots.org is good, but some generator nodes can be extremely slow (30 minute wait times). For IE-only screenshots, I like to use http://meineipadresse.de/netrenderer/index.php , which provides IE screenshots in about 7 seconds.
Litmusapp may be nicer and have summaries, but it's hard to beat browsershot's simplicity, breadth, and free-ness. Though I have yet to see any provide decent testing for anything dynamic (videos, perhaps?).