I would think gedit would be perfect. It is on Ubuntu by default, is easy to use, and can be extended with plugins.
That being said, since notepad++ is based on scintilla, scite is probably worth checking out. Scite is based on scintilla as well, though as fred.bear points out, it might be less feature-rich than you need.
Here are a couple more:
jedit - A plugin-based editor for programmers
jEdit is a powerful open source editor
[...] unlimited undos/redos,
syntax highlighting for more than 130
file formats, markers for fast and
easy navigation, online help, bracket
matching for programming languages as
well as auto-indent. Plugins can be downloaded and installed from within jEdit using the "plugin manager" feature.
vim / gvim / cream
gvim or cream - VI IMproved, with optional VIM macros that make the VIM easier to use for beginners