I have four prototypes; two NES and two SNES. I don't know what the NES versions are, but I suspect they are the unreleased AVE Backgammon and Cue Ball. The SNES prototypes are a SNES MusiCart and Bart's Nightmare. Gameplay of the Bart's Nightmare prototype can be found here:

Ikari Warriors and Roger Clemens' MVP Baseball. Bought 10/28. Paid $200 for Ikari Warriors and $100 for Roger Clemens. Have dumped Ikari Warriors and found differences compared to a dump I did of the production version. Cannot yet seem to get Roger Clemens dumped. Ikari is labeled as Sample Cartridge, but contains 1 EPROM and 1 ROM chip.

http://imgur.com/a/vBttc

I dunno if this is still being updated but I've had a proto cart for Hook on NES for a couple months now.

http://imgur.com/a/0rPQ7...

Bump. It looks like the thread may need to be re-sticky'd.

- Heavy Weight Hockey (unreleased/undumped)

I don't know yet if there are any differences between production versions and these others:

- Burai Fighter
- Baseball Stars
- Journey to Silius
- Micro Machines 
- Super Sprint

Please put me on the list for a Solar Jetman proto. I purchased this from MrMark sometime last year. Thanks!

have another one I can't get to boot, but would like to be added for these 4 titles.
Tengin Tetris (playable, diffrent start screen from what I've seen)
Conquest of the Crystal Palace (Start screen reads "Lord of demon paradise ")
Wurm
NES play action football