Being a Linux/Ubuntu user, i cannot use the praised Textmate editor to develop ruby applications. I tried a lot of editors, some of them being very good ones, but in the end I always end up using gedit.
gedit is great… but it is rather limited features-wise. Recently I have tried Komodo Edit and I can say that this is the best RubyOnRails editor available on Linux. It has a lot of features (autocomplete, tons of supported languages, macros, color schemes, etc) but somehow it is still light and minimalist.
Komodo Edit is also available on Mac and Windows