I've just written a new Emacs mode for Rails development. I know there's already a rails.el, but it seemed like too much for me. For one thing, it assumes you're going to be using mmm-mode. In my experience, mmm-mode is a huge pain. If you make changes to a buffer, you have to manually re-parse the buffer to get the syntax highlighting to reflect your changes. It also assumes any ruby files you're editing are going to use rails mode, which is absolutely unacceptable.
So I scratched an itch: arorem is Another Ruby on Rails Emacs Mode. It's much leaner than rails.el, skipping out on a lot of the stuff I don't use at all. (webrick management and documentation search, for instance.) It's got a dedicated rhtml major mode rather than trying to mix major modes on the fly via mmm-mode. Anyhow, more details are available on my trac. I'm very interested in seeing what other people think of this as it's my first nontrivial piece of elisp, so if you have anything to say please edit the wiki or file a ticket.