I recently created and uploaded two new plugins that seemed sorely needed for UI. I looked all over and couldnt' find ones that fit my needs, so I just bit the bullet and wrote them myself.
2. Collision events during dragging, as well as collision-avoidance during dragging:
The source-forge pages have wiki's with documentation, and the downloads have several examples embedded.
Feedback is welcome, I'm new to jQuery.
--Sean