Thanks - I really like the categories your site has for each thing that you post. I think it'll be pretty valuable to know whether you're quoting from a tweet, a book, or a newspaper article, and not only depend on the source domain.
Also GitHub Link: https://github.com/ideonexus/memexplex
I know there are some other options posted, so I figure I'd throw my own side-hobby out there http://mxplx.com/ is an open-source project I wrote to keep track of my favorite quotes (memes), link them to references, triple them to other memes, and gather them in lists (schemas). I never got around to adding a clipping-tool to it, but I did interface with Google Books to make gathering book references easier.
It's unnecessarily erudite, ugly, and I turned off new signups so you'd need to download the source from github or google code if you want to play with it (I do include a copy of the database complete with my personal collection of quotes and references on Git), but I'm posting it here because someone might find something of interest in it.