Text editor with syntax highlighting, theme support and code folding

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

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

Without GUI or with a simplified GUI (still needs to be run in terminal):

• vi

• vim

• nano

• ed

• pico

• emacs

• xemacs

With GUI:

• geany

• mousepad (default under Xfce)

• kate

Please note that some of these programs may not be available from Ubuntu's official repositories.

Try Bluefish, Geany, or even installing Notepad++ trough Wine if you want.
More on installing Wine on Ubuntu and one example of installing some application using Wine.