In my projects I prefer use lists of checkboxes above select multiple. So, I extends XoopsFormCheckBox class to render this approach: http://www.1stwebdesigns.com/blog/development/multiple-select-with-checkboxes-and-jquery


But I was looking for additional improvements to the very long (filtering, autocomplete, etc.) list of options lately.
I have found some interesting jquery widgets.
http://loudev.com/
http://quasipartikel.at/multiselect/
http://harvesthq.github.com/chosen/

Pros:
- at first glance you can see what options are selected (there is no need to scroll through a long list to find out that two or three options have been selected)
- replacement for the standard with multiple attribute on browser side (no need for major changes in the modules code)

Cons:
- sometimes there is no possibility to enable javascript (or script errors because of eg. updating/changeing browser in future - developer must take an action). So you are left with a standard which is worse than a even simple checkbox list mentioned at the beginning.

Are you using any other solution?