Tractionbeat lets you see annotated Google Analytics charts of other startups, so you can learn how they built up traffic over time.
Awesome to see traction for some new startups, especially if they add annotations. I've added Headway and got some nice feedback and suggestions from random people about our traction progress.
Annotations are a great insight to what others can do to get better stats
If only more sites added annotations
Was one of the beta users of this site and really like it!
Easy to use, nice styles, annotations are fun, open startups for the win!
Nothing immediately comes to mind.