I love the "Tags" textbox on Stack Exchange. It looks like a hassle and a half to write though, so is there a free "label textbox" scripts I can use?
Alternatively, is the one on Stack Overflow free to use?
While we wrote the tag editor from scratch, it's heavily inspired by the "Chosen" plugin. Have a look at the "multiple select" examples on that page; it may just be what you're looking for.