Tomboy looks decent. You have decent formatting capabilities, linking, a handy table of contents and some plugins to extend the functionality. But what is missing is the ability to embed images.This lead me to Zim.
Zim tries to bring wiki-style to the desktop.Which means you can can use wiki-style syntax  in your notes to full effect. It really is handy when you can use those underlines and TODOs..plus all the other things-formatting, links, plugins are there as well. Images can be embedded as well.There is a takeSnapshot plugin which i liked a lot! it directly embeds a snapshot in your note..very handy at times!
Snapshots are useful when i want some part of webpage and put some relevant notes alongwith it.
The install instructions  given on the site suffice. Some plugins need additional libraries, etc which is mentioned wherever applicable. The notes are stored as simple text file with .txt extensions. For now, i m finding Zim to be really good. Images, snapshot and wiki-syntax is where it scores over tomboy. Anyone with any better alternatives?
P.S. I am not good at writing technical posts. This is a feeble attempt after a long time! Hope you find the app useful!