There's also the lesser drow. But I'd put Darkmask no higher than +0. You have to be drow and have to spend 2 feats and unless you somehow get move silently as a class skill, you can't meet the prereqs before level 9. You'd also spend 32 skill points (8 hide, 12 move silently, 8 sleight of hand, 4 survival) on the prereqs, 20 with able learner. How many skill points does the cleric get every level again? This is not for clerics. (Wilderness) Rogue 1/Cleric 4 or (Savage) Bard 1/Cleric 4 could do this, but the loss of a caster level isn't worth it. My suspicion is that it's a low -1 for cleric, archivist, or anything that isn't a Divine Bard (the latter could be a low +0, I guess).
Agreed on Thayan Knight (which is identical to the one in Complete Warrior).
Spur Lord: Yeah, sounds about right.
Skymage: What? It does take 3 feats to enter, but it gives 5 feats back: Craft Wand, Spell Focus, Skill Focus, 'Flying Feat' (Flyby Attack) and Enlarge Spell. It also gives the benefit of the Collegiate Wizard feat for those five levels to Wizards, and free scrolls otherwise. Not to forget the mount. AND it is full casting. This is +1.
Thanks for getting the ball rolling, though.