A custom field control for WSS 3.0/MOSS 2007 to get filtered data from other list based on complex queries that is otherwise not possible through OOTB lookup field.
This is one of my older projects having two custom field definitions:
1. the Parent-Child control (improved on the Datacogs solution)
2. Query Based Look Up Field Control
Query Based Lookup Field
Parent Control
Child Control
for related posting:
http://baigadil.blogspot.com/2007/11/sharepoint-2007-custom-lookup-drop-down.html