Mines make credits.
Prostitution makes a lot of credits. (Takes a bit though. The reputation system is bad)
Selling items. Can get a lot by tweaking the npc bondage rate in the encounters menu. Specifically the My fetishes and Goal Weights.
Stealing Apples.
Selling items from defeated npcs.
Mods, like The Plasma Cafe.
Slaves - Both selling and Prostitution.
Experimenting with Eliza Quinn in medical.
These are off the top of my head.