I'm creating my own form based on this and I'd like to have a tag selector which is similar to the one on StackExchange. I can roll my own but I was wondering if something similar already exists.
you can do that using JQuery autocomplete plugin
and once you have included all of the needed JS files just add this code after your new post form
note: Now this is good if you only have a small number of tags, but if you have hundreds or thousands of tags then using an ajax solution is a must.
Look at the code for the tag box in