I have recently been using a tool called Evernote. At first I thought it was just another online tool but the more I use it the more I love it. It has four ways to get to it. I have a client installed on my PC, a client installed on my Mac, there is an iPhone application and a web tool.
It allows you to take notes in various notebooks - similar to OneNote - and it allows you tag those notes. You can also email those notes. Fairly basic features. The cool part to me is the iPhone app. It allows you to record voice memos, take pictures and make notes. All of these can be tagged and put into specific notebooks. All of this information syncs over the web and all of your clients can see the same thing. This gives me a way to compile data all in one place and see it where ever I am.
One of the other cool things is that it will index and search on text that is within pictures. This works fairly well but not perfectly. If it is typed it works pretty well and finds some hand written stuff.
This has become my main note taking tool in meetings and when a whiteboard is used I take a picture and sync it with Evernote. I also make notes to myself using the audio record function while I am in the car.
This is a beta and free right now. There is a premium version if you exceed their data limit each month which seems to be fairly large. I have not reached it. I would pay for the premium service at this point. They could take this to the next level by adding a task list and allowing you to share this with others. I would love to make this more GTD friendly. Still I am a big fan.