This piece on TechCrunch talks about how Optimizely is rapidly catching up on industry leader Test&Target.
In the comments, an Adobe product mangler points to a blog where they discuss unnamed competitors. Optimizely follow up by throwing down a challenge: try Optimizely and Test&Target side by side and decide.
If you haven't tried the Optimizely demo, well worth a go. It's awesome. Love the product. Be warned though, the pricing isn't the Enterprise Grade you're used to ;)