Recommendations for HTML/Ajax multiple select with autocomplete library or plugin

Question

I'm looking for a library or jquery plugin (etc) for a dynamic, multi-value form selector. Think of gmail's "TO" field when composing email.

Looking for:

• Populate field(s) with 0 to n employee ids.
• Currently have 5000 employees.
• First or last name searching.

I've rolled my own in the past. However I'm trying to focus my development time on specific business problems.

Thanks.