Specifically I'm interested in games that are designed for solo play or have solo play possibilities baked into them (Four Against Darkness, Dungeonous, Scarlet Heroes, ...) rather than solo engines to use with RPGs in general (Mythic GME etc.) or gamebook systems (Lone Wolf etc.).
There's a good number of them out there and currently my time constraints don't allow me to simply try out all of them.
And I forgot the fantastic The Skeletons, by @jmstar, which can be played solo. You don't write anything down but there aren't any dice either. Also available from DriveThruRPG.
Possibly not that interesting for you due to the lack of crunch, but I think it's a great concept, maybe you can try a similar reversal with whatever game(s) you end up with.