Want a quick way to jot down linked ideas - make an online brainstorm or mindmap. The take a look at bubbl.us
The concept is pretty basic: you write your thoughts in bubbles, which you can then connect to other bubbles, eventually creating as complex (or not) diagram as you could possibly want. It's a great way to quickly jot down and map your ideas; and you can create as many of these as you want.
Also consider thinkature
t's free and easy to use - via Lifehacker