Try pg's list of "ideas we'd like to fund". http://www.ycombinator.com/rfs/

Ancient, but here's a blog post I wrote with some ideas: http://www.puremango.co.uk/2010/10/ten-ideas/

And Jacques Mattheij's post that inspired me: http://jacquesmattheij.com/My+list+of+ideas+for+when+you+are...