I did a search and couldn't find any other posts about this, so I thought I'd share my find.
This is a database of used lens prices, which looks to get regular updates.
It includes Canon and the non-OEM lens manufacturers. Once you read the instructions on how to decipher the chart, it can be comprehended quite easily It's all in USD, even though the site is Canadian.
So for all those posts asking "how much is my lens worth/is this a good deal," take a look!