custom multi-select fields

I have a couple of projects where it would be useful to have custom fields that are multi-selects. There are various ways to implement this. Use cases include indicating say, countries of the world where an organization operates, principal sectors of a list of 100-200 that it works in, issues of interest to a constituent, etc. A current (ugly) hack would be several identical single select or option group choices, perhaps with special code to ensure they aren't the same. The problem would come when trying to specify search criteria for what should be a single field.

K: Might be a good project for deepak/manish, since this is similar to checkbox code