A try for an AJAX-powered Del.icio.us-like tag suggestion (or also the gmail "recipient for your mail suggestion" function) with the Prototype javascript library.
The code could be easily much more optimised I think...
Tested with Firefox 1.5 and IE6 on Windows.
Type several tags in the input field, separate each one with a comma, then select a suggestion (select with arrows and enter, or click).
Ex: apple, banana, orange