Great Ball Contraption

Balls on Bricklink 2022-10, $2.00 each, ouch!. An off-brand solution at a much more reasonable price. I've never tried them, but some internet people have had good results.
Electric, Train Speed Regulator 9V Better than batteries. Also, for those of you who may be interested in using the train controllers for wall power, you can use any 12VAC wall adapter with at least 1A output on a 5.5mm plug, e.g. this on Amazon. If you have got a collection of wall warts from random dead electronics in the past, you may already have one of these lying around. Total cost to get the controller & power supply is under $20 (still, in 2022).
Connector wire To go from 9v train speed regulator to power function motors. Sadly out of production, updated the link to brinklink.
Great Ball Pit A regular blog that supports the GBC online communuity; a bit dormant during the pandemic. Also a source for an invite to the Great Ball Pit Discord, an excellent community of builders.
greatballcontraption.com One stop shopping for GBC information.
www.brickwiki.info/wiki/Great_Ball_Contraption A wiki with forums
www.teamhassenplug.org/GBC A useful definition of the standard
Gear placement This guys site is a excellent resource for building mechanical models
Bricklink This is the source for individual pieces, when you know what you want.
peeron reference for what parts are in what sets.

My talk about GBCs, and the revised directions for a Push-Arm GBC module. Watch this space for updated directions, but probably not until January. This talk is strongly based on my 2014 one
Rolling in the Pacific Northwest my presentation on GBC's from BrickCon 2014. A discussion of what a GBC is, and how to approach them. (The power standard is out of date; I now use 3 bricks high)
Slides from my Introduction to Great Ball Contraption talk at Bricks Cascade in 2017.
Slides from the intro talk at Bricks Cascade 2018
Introduction to GBC slides from the talk at Bricks Cascade 2023

Bricks Cascade GBC Kit 2018 Directions and the parts list
Directions for a simple GBC Push arm moves balls up a ramp. As seen at BrickCon 2015, Bricks Cascade 2016, BrickCan 2016... Slightly updated version available as part of the 2016 BrickCon talk
Revised Directions for the Push Arm GBC, using the power standard instead of a Motor. Likely to be a kit/workshop at Bricks Cascade 2017. Maybe 2018... Parts list for this version.
White LifterA conveyor belt that also works as a power source for shared power
Forum thread on EuroBricks.com with directions for making some of Akiyuki's builds
Brickworld Ball Pump The 2018 kit directions; a remarkably reliable ball pump. I experienced some occations when it failed to feed, but it never jammed, and presumably would have eventually stirred a ball into the right place. Or maybe I did the directions wrong. A solid beginner pump for people with the required parts.