I really like developing with Rails but for some types of web applications Merb is a very good fit. The Merb framework is much smaller than Rails and you have a lot of choices for ORM, template engines, etc.

My DevX Merb article was just published this evening and provides a simple example: rewriting my RubyPlanet.net Ruby news filtering web application.

I find Merb to be complementary to Rails so if you like developing with Rails I think that you will like Merb also.