I've recently switched over to Ubuntu from Windows and love it. One thing I miss is my Programmer's Notepad. I haven't found anything similar yet for Linux. Eclipse is way too big and involved for what I need.
EDITED TO ADD:
Specifically I'm looking for -
a diff tool (I know about Meld, but am looking for built-in or plug-in rather than a separate tool);
and the big one - find in files - search for a term or regex in a user-specific group of files, or recursively through directories, and return a highlighted, clickable list of results.
Edited again (04/05/2011) I did end up trying most of the suggestions below, but what I ended up with is Komodo Edit. It does everything I wanted, and it's available on all three platforms, so now that I'm on a Mac at work, I don't have to learn another new IDE. It's build on Mozilla, so there are add-ons (and you can create your own) which can be updated the same way Firefox add-ons are.