Here's the Inventor COTS parts library I've put together over the past few years. I've downloaded a variety of STEP files from AndyMark and Vex and converted them to .ipt / .asm files. It has a common set of gears and motors and such instead of every part having its own copy of a particular gear. It is not perfect (the fasteners are a bit of a mess for example), but there's good stuff in here.
http://www.nemoquiz.com/files/COTS.zip (300 MB)