UPDATE: A !! new version !! of this guide is avalaible.
UPDATE: This guide was for gedit 2.18.x, please upgrade to gedit 2.20.x to have an easier color scheme managing. Textmate is really a great text-editor (almost an IDE but faster), it has even the support for latest Ruby/Rails technologies like RSpec. On the GNU/Linux side there are also similar full-featured text editor and one is Gedit.
Gedit can be extended by plugins written in C or in Python, here is a tutorial how to set up a Textmate-like Gedit.
By default Gedit supports a lot of languages php, C#, java,.. and Ruby, but Ruby embedded. To enable the sintax highlight in HTML files with embedded ruby code (.rhtml) you have to add 2 files (Thanks to Bitsbam).
From the terminal:
$ wget http://grigio.org/files/x-rhtml.xml; sudo mv x-rhtml.xml /usr/share/mime/packages
$ wget http://grigio.org/files/rhtml.lang; sudo mv rhtml.lang /usr/share/gtksourceview-1.0/language-specs/
The list of the plugins avalaible is growing fast; this is a list of plugins that work well each other.
Snippets: it permits to call structures of code easly and everytime you press TAB the cursor automatically jumps to the right position
Snap Open: it opens the files that contains the word you are typing
File Browser: a treeview of the files in the current directory
Word completion: it complete the word while you are typing and with TAB you select it
Class Browser: a list with all the methods in the current file
Quick Highlight Mode: very useful to specify quickly the syntax mode
UPDATE: My color scheme is here (It's ONLY for gedit 2.18.x), uncompress it in $HOME/.gconf/apps/gedit-2/preferences/syntax_highlighting and restart the session. Be sure that the folder exists and that your previos files are overwritten.