I just found out one agent is now on Twitter. It's Donald Maass. I was lucky I got to attend his Breakout Novel workshop with a bunch of writer friends a few years ago. His workshop was great.
Follow him at @DonMaass
He's been twittering daily #breakout prompts
Here's an example:
For your MC, what are the best things about these times? The worst? Create a passage of his/her take on this era. #Maass
If you haven't read the book, do! In the meantime follow him and pick up some great prompts.