I've recently gotten into 3D paper buildings and whatnot. One site I encountered looking for patterns is Papermau. Papermau has original designs but also aggregates 3D designs found elsewhere. Some of the links are old and no longer work. As always when dealing with the interent, be careful before downloading anything. Papermau uses Adfly and Depositfiles for their personal design downloads, which have a lot of adds and confusing download options. I've not had any malware issues so far, but be smart and be careful.
The top of the page has some general categories, if you want more specific searches (for specific model scales and categories) scroll down pretty far on the right.
Some of the models are hand drawn, others computer generated. Some are black and white prints, others full colour. The quality varies, but there's some good stuff there, depending on what you're looking for.
Most of the files are full colour prints, which aren't as good for what I'm looking for, but if you play D&D and want some models... this site's got you covered. There are several castle builds, churches, medieval villages and towns, knights, pavilions, a trebuchet, etc.
Here's a pattern for a Joker paper doll.
There are a ton of other interesting patterns: old mini arcade games, Japanese armour and castle, a celestial globe. There are robots, anime figures, SF character and ship models. It's an amazing collection.
If you want some amazingly detailed and realistic models, check out this site.