Threads AJAX post roller (like DiggSpy)
 

My absolute favorite vB mod/feature is the AJAX post roller. MPDev has a version of it here:

http://www.vbulletin.org/forum/showthread.php?p=1085784

I run it on all of my sites* my members use the hell out of it* and other sites have implemented similar versions of it as well. I believe the first one like it was Digg Spy:

http://labs.digg.com/bigspy/

If this could be a default feature of VB4* it would be fantastic. It's certainly extremely useful for any site.