wBrain Slice is a free real-time tool to create discussion boards for remote and collaborative teams.
Just a clean area to throw some ideas. Nice way to start!
If you need a more traditional way to make retrospectives, go for it!
SWOT, is-isn´t-does-doesn´t, good-challenges-ideas-recognition... a lot can be done with quadrants!
You need more? Check out these boards templates!