A while ago, there was an Eclipse plugin called RDT (Ruby Development Toolkit) that provided "simple" Ruby support (a nice editor, some code completion etc.). Development of this plugin was discontinued and instead rolled into Aptana's large RadRails suite since RDT's developer was hired by Aptana. This obviously was great news for him but sad for me because I didn't want the heavy RadRails environment with its hundreds of plugin jars doing everything from HTML to Cloud Computing deployment. Given Eclipse's pluggable nature, I figured that I should be able to take a RadRails install and only extract the portions I wanted to give me my basic RDT back except now it's the latest and greatest.
Below is a directory listing showing the directories and jars to extract from the RadRails install to make your eclipse have Ruby development capabilities. I found this through trial and error and they seem to work just fine but if you find errors, post comments for others to see.
I highly recommend creating a separate extension location to store these plugins so future upgrades become easier since you know the features and plugins in this extension location are solely dedicated to RDT.
I am not a serious web developer requiring me to install the full Aptana suite, which significantly slowed down my other development in Eclipse, so this little trick has worked well for me. Now I have my RDT back without all the unnecessary plugins!