Note organizer with tagging

[Enjoju]

I'm trying to reorganize my dissertation notes, and need a note organization program similar to Ubernote (http://www.ubernote.com/webnote/pages/checklist.aspx)
that will let me:

-group notes by categories
-tag each note
-allow me to search and/or retrieve entries with the same tags

I'm currently using Keynote, so any program that does a tree-structure is basically what I already have. Also have OneNote, but I don't like how it handles tags. Will use it if I have to.

For instance, I want to be able to do this:

Name of entry-Smith 1999
Tags: Field A, issue B
Notes: (abstract, my notes, quotes, etc)

I am a poor grad student and can't really afford to shell out money for programs like Citation that do this.

Thanks!