I'd suggest checking out Mach-II. It's used on numerous extremely high-profile, high-traffic sites and has been for years. This blog post is a bit old at this point but still has some great info in it in terms of Mach-II in general as well as who uses Mach-II.
It's under very active development (don't let the lower version numbers compared with other frameworks fool you) with version 1.9 just around the corner and big plans for 2.0.
Big features of Mach-II include sophisticated caching and logging functionality, a very nice dashboard for helping with development and monitoring of your applications, a rich form tag library complete with data binding, and tons more.
