While working on CrissCross I've experimented with various different jQuery MultiSelect widgets. That is, web page controls that let users easily select multiple items from a list - here's a quick round up:
Eric Hynds jQuery UI MultiSelect Widget
This one is rendered as a DropDown and I think overall its my favourite, it has ThemeRoller support and there is an extra plugin that lets users do a search within the list. Here's what it looks like while 'dropped down':
Quasipartikel jQuery UI Multiselect
This one renders as a 'two listbox' layout and also has ThemeRoller support. Looks like this:
This one has an ingenious design that makes it a lot more lightweight than the ones above.
The Odyniec selectList just adds a few simple components onto an existing Html Select element to make it into a multiselect - that means it takes much less time to render.
Basically, it adds an interactive list of chosen items under the original Select, like this:
LoopJ jQuery TokenInput
The TokenInput is a 'tokeniser' - you type a bit, it shows matching results and then you choose one. Then you type again, it shows matching results again, etc. This is the sort of control that is used for selecting 'tags', but it also works well as a general Multiselect if you have a very large number of items. Users dont have to scroll through the list, they just tap a few keys and then select.