I'm finding OmniOutliner to be extremely useful as of late. I never really considered usng my powerbook to take notes in class until I discovered it came installed on my powerbook. The OmniGroup/OmniOutliner is another perfect example of why i'm liking the OS X era Apple "platform", as opposed to say Windows or even Linux.
Generally, there are two options for taking notes in a lecture type setting when using a laptop. There's the basic text editor, be it vim, emacs, pico/nano, whatever and there is a word processor, usually MS Word.
While MS Word, AbiWord, Open/Staroffice, etc... have built in facilities for building outlines, they are primarily aesthecally driven, for the purpose of formatting text for presentation, not necessarily producing something that is functional or even useful. I remember having to go through the steps of producing an outline for a professional communications class and it was damn near painful, it's hard to predict exactly how the word processor is going to handle the outline especially after the fact. In addition, that's alot of processing overhead for something that doesn't do a very simple task very well.
While the standard text editor eliminates the bloat and unpredictability of a word processor, what you end up with is an outline that becomes cumbersome when you start having to go back and make changes. Also, (while more than the word processor) what you end up with isn't very functional.
OmniOutliner solves both problems. There is relatively low processing overhead to do one thing very well to produce something that is very functional. I've found it very easy to use on my powerbook as it can be easily controlled without the use of a mouse, which helps to keep up with the professor. It's hard to understand how truly easy it is to use form the keyboard without actually using it... cmd+] to indent, cmd+[ to go up a level, up/down arrows to navigate cmd+left/right to expland/collapse a tree and return to add a new row. What you end up with is an outline that can be navigated in the expandable/collapsable tree style in a number of formats, including the default Apple style plist XML serialization format, Rich Text, HTML and ASCII. The advantages of using the Apple Property List format is that functionality is built into the cocoa framework to interact with plist with ease so external apps can make use of the outlines produced by OmniOutline without alot of work, and since it's standardized XML, it'd be relatively easy to interact with the data from any programming language. Not only that, the application itself is scriptable via AppleScript. OmniOutliner also supports many of the formatting that one would expect, like various numbering conventions.
It's not Open Source, but that only becomes a problem when the program has shortcomings, right? Besides, The Omni Group is very develoepr friendly, when it comes to providing the developer with the tools he/she needs.
Anyways, just spreading the word. I recommend anyone who has access to a mac to check it out... and if you don't have a mac, regardless of you're GNU/BSD/Linux/UNIX/Windows tendancies, get one!