vim-rails now has basic support for merb.

Mini HOWTO:

$ cd /tmp
$ git clone git://github.com/tpope/vim-rails.git
$ cd vim-rails; rake install

Now the plugin is installed, but it needs a small workaround to detect your merb app:

$ cd $MERB_ROOT
$ touch config/environment.rb

There are some useful scripts for Vim :

• Merb, Datamapper and jQuery
• Haml
• Sass

If you’re using TextMate for development check out the following bundles:

• Dr Nic's Merb TextMate Bundle
• HAML/SASS bundle

you can configure gedit to function in a very similar manner

• gedit gedit config

There are a lot of IDEs with ruby support. Explicit Merb support is not available in any of them.

• Netbeans Merb support coming up!

• IntelliJ Idea
• RubyMine

Both products are commercial.

• Aptana RadRails
  • Developing for Merb in Eclipse/RadRails
  • Note: I find Aptana's Ruby mode to be more helpful than its Rails mode, personally — Max Aller 2008/11/16 21:16
• Eclipse DLTK project features Ruby support.
• CodeGear 3rd rail - commercial product