Kiel Figgins has an awesome collection of rigs, from vehicles, to creatures, to realistic humanoids.
This one's a bit tricky to post (in terms of individual rigs), so I'll post each rig set separately. I'll put down labels for each though for the rig list and search function.
I've used the jet/chopper type of rig (rig set 6) and the mecha walker (MechV4 / rig set 1 - currently featured on the Animation Buffet Banner) and they're great. Once my workflow tutorial is done (don't laugh!) I can put up reviews of those two rigs.
Head over to 3dfiggins.com for the rig sets and plenty more.
Rig Set 1 contains the rigs: