Ruby Gem for intra-site messaging? (think LinkedIn, Facebook, etc.)

Many sites have communication via the site (instead of via email). Examples:

• LinkedIn: Click "Inbox" at the top of the screen.
• Facebook: Click "Messages" in the left sidebar.

开发者_如何学JAVAIs there a gem that provides this sort of functionality? Googling hasn't turned up much, because the search terms are so generic - "Rails gem messaging" "Ruby gem email" etc. :)

Thanks!

There is an old plugin, another old plugin and a relatively fresh one.