I am a great admirer and supporter of initiatives, including but not limited to websites, that endeavour to explore and refine ideas. Voyaging through uncharted possibility space, especially in a collaborative setting, is one of the really great ways to while away the hours on this planet.
A UK-based repository of social innovations, voted on by users for originality, feasibility and humour. (A nice example.)
A sandbox that is home to a lively, whimsical castle-building sensibility. (A nice example, which suggested to me that they could feature a long-term thinking section to help generate interest and ideas around projects that aim to encourage foresight, Long Now-style.)
The following also look interesting but I'm less familiar with them:
ThinkCycle: Open Collaborative Design http://www.thinkcycle.org
WhyNot? Idea exchange http://whynot.net
Premises, Premises: A peer-enforced marketplace for new ideas http://premisespremises.com