Well, originally it was about doing 3 different takes on sci-fi using the same basic rules, with a few basic moves swapped around. Black Seas of Infinity is essentially Evil of the Stars, but in a social context (not isolated) and with the PCs being farther along the badass cyberpunk spectrum. It's supposed tohave 4 different optional setting add-ons (like 1 for social networking rules, 1 for psychic powers, 1 for building machines, and 1 for eschatonic symbiotes), but I've only got stuff written for the first of those and I dunno how well they work.
And Debtrunner is basically Traveller, but focused more on the map-building rules and speculative trade rules from original Traveller, which I wrote more complicated versions of. Uses mostly the same basic moves, the vantage thing, plus the social networking rules, but no stress or injury tracks. It kind of moves farther away from the other two the more I work on it.