These plugins work with Motion 3 and Motion 4.
ProAnimator from Zaxwerks for 3D Titles and Logos 
Particle Metrix from Idustrial Revolution. Here’s a review. 
Toon-A-Matic from Sheffieldsoftworks 
Numbers generator from Tapio
Tapio's full FxPack 1.1 including an animated light sweep
Motype from Yanobox 
3D Motion tracking plugin from PFHoe 
FX Factory 
Boris FX Final Effects Complete 
Motion plugins list at Toolfarm
Idustrial Revolution Volumetrix
FxFactory
Here's a review dated 10/10/07 
AutoMotion from Digital Heaven
Sapphire plug-ins from GenArts 
Continuum Complete from Boris FX. Here’s a review 9/09. 
Shine and Starglow from Trapcode 
Particle emitters from Wondertouch 
Pro Import AE 3.0 from Automatic Duck 
dvMatte Blast real-time dv/hdv/hd keyer from dvGarage 
3D Flag and Stackwerks from Zaxwerks 
CHV Electronics FxPlug Plugins
Some excellent tools from Special Case:
-The Collector is a great little tool for collecting and optionally zipping up a Motion project and all its associated media.
-A script for exporting an Illustrator path as a Motion shape object file
-A script for exporting an Illustrator path as a Motion Path behavior preset
-A script that exports the position keyframes of the currently-selected layer as a Motion position preset. This allows for AE-applied tracking data to be used in Motion
-A layer preset that makes video footage look like old film