I just started http://opencobol.add1tocobol.com/#where-can-i-find-open-cobol-source-code and if you know of systems, please add to this thread, and I'll plop in a reference and blurb. If it's open source COBOL, size doesn't matter. All the odds and sods.
I'll add the add1tocobol links shortly, and repeat the links peppered through the FAQ, but the more the merrier. And things like Trek, and Hamurabi and WUMPUS!
The following link also contains COBOL source code, which I am busy cleaning up: https://sourceforge.net/projects/apac-accounting/.
I am currently busy with changes, including some additional remarks, changing layouts of screens to make them more understandable. Some of the new files are in the SVN in the New Development folder. These files will be updated frequently over the next few months and the entire system should be available in this folder by the last quarter of 2013.
Hope this helps the cause.
Added. Thanks James.
Wim Niemans has created a portal to help boost open source COBOL. Project Stickleback.
A small website: http://stickleback.nlbox.com
A portal: http://www.myCobol.net
Check it out.