Facebook got tokenized input right. If you've ever sent someone a message, or searched for someone a message you probably used what was previously a less user-friendly system to perform this same task. The idea is easy- you want to enter one or more names or other text and select matches. Gmail uses the same kind of auto-complete to allow you to email to multiple email addresses, as do a host of other sites, so it's surprising there aren't better solutions to this in the public. I came across one amazing solution for this when I was working on MovieFly a few months ago, and was able to expand it with a few more options while working on SponsoredTweets .
The jQuery Tokenizing Autocomplete was released last year, with some nice additions to it in December. It's an extremely rich plugin that behaves very similar to the Facebook version. You can select previously entered tokens and delete them, navigate completely by keyboard, or with the mouse- it's amazing. My branch has a few additions that were used on SponsoredTweets including suggestions when there are no matches.