I've had the same problem. I tried using Basecamp, Trello cards, and spreadsheets, but none really helped organize my thoughts how I wanted. What I did want was somewhere I could define ideas, define tests / hypotheses, and collect feedback so that I could see all this information in one space.

I ended turning this problem into a side project (leanlab.io) which you could checkout ;). I also know that Javelin are doing something similar with experiments (experiments.javelin.com).