|A discus and neon tetras from Toucan Virtual Museum, rendered with POV-Ray|
The places to go for models (in no particular order).
This is probably the biggest resource of user-submitted 3D models in Internet, ranging from free or cheap class works to very professional and expensive characters and architectural models.
2. DAZ 3D
DAZ is the creator of both the professional 3D applications and the base human characters for Poser/DAZ Studio (Victoria, Michael and various derivatives). They have online store with lots of high quality models, some of them available for free (require registration still), including the starter versions of most of their human characters.
3. DAZ 3D forums
They have special sections for advertising free models. From there you can find links to some free hair and clothes to complement your figures.
For many this is The Internet Marketplace for 3D art. Huge amount of models for Poser/DAZ including many free ones. The prices seem to be a bit cheaper than in DAZ store. Registration required.
5. Toucan Virtual Museum
A small site that has some very nice free fish, flower and insect models, that are hard to find otherwise.
6. Archive 3D
More than 20 000 architectural models (mostly furniture) available fro download. No registration required.
7. Google 3D Warehouse
The companion site to Google SketchUp free modeling program. Big and growing selection of free user-created models.
8 Renderotica (Warning: Adult Content)
A must be site, if you want to render some steamy action. No free models, but otherwise good selection of characters, poses and accessories.
I have not tested them all, so I cannot tell, how much Khayyam compatible model formats are there.
Have fun! And do not forget to support the creators by purchase, donation or advertisement, if you find their models useful for you!