3e-Tome of Battle: I'm mining it for Martial options. I think a combination of its Maneuver/Classes could be made into a cool series of subclasses, possibly even shared between various classes. Crusader as a sub-class for Paladin is easy enough, Warblade goes to Fighter, and maybe Swordsage as a shared Rogue/Ranger. Shrug, it's something in the back of my mind at any rate.
4e-I'm currently mining it for all it's worth. Playing with lots of mechanics ideas to spice up 5e, including some form of "Encounter" spell casting. It'll probably end up looking like "Spell does half damage, on failed Saving Throw it does no damage, effect persists until the start of your next turn" or similar. Also working to bring back more options for Martial characters from it, as well as the four defenses and marking.