Most recently, a couple of rolls of the old J & C Classic Pan 200.
I bought 60 rolls of this a few years ago, for camera tests and the like. IIRC it was about $1.00 per roll on sale.
Recently, I've been testing out a bunch of used Mamiya RB film backs, as well as an old folder.
This has meant that I have a whole bunch of negatives that look okay until you have to try to work with their curl!