A more lightweight replacement for Tomboy is Zim. Like Tomboy, it's a desktop wiki in that you can automatically create links within notes. Largely, though, it feels like a much improved notepad.
Zim works like Tomboy in almost every way. Possibly the only thing it lacks is the synchronization feature. But I don't use that anyway.
Here are the things I like about Zim over Tomboy:
Overall, it feels like a good mix of Tomboy and Gedit.