I have been playing and enjoying Roland's Soliatire. The one that is the first on the list from the PDF solitaire variants.
Yes, this version does without AI. Really it is great mix of "regular card solitaire" and R4tG.
By drawing from a pile of all 4 player role cards, you know that there will be 4 of each action, which makes for some very enjoyable decisions: when do you trigger each production, when do you consume. (especially given you have 8 consumes!)
There is one thing that bothered me...knowing there is only 4 developments and 5 worlds (4+homeworld). I found it hard to get a full strategy going (especially given the lack of production), and you end up with a ton of cards in your hand that just get discarded, as you run out of placements.
I made a tiny adjustment: add 2 develops and 1 settle from from the 2-player role cards. This will allow you to build a full tableau.
the scores will be much higher, maybe artificially so, but it just felt like a more complete, satisfying game to me. but kudos to Roland for such a simple, addicting variant!
I have yet to try other versions, as this one really hits a sweet spot for me. But I will give Jeff's ballyhooed AI version a shot too.