http://en.wikipedia.org/wiki/List_of_leaf_vegetables
This covers most of the fast, short-term annuals with two caveats:
1) There are longer term leaf vegetables like Cardoons.
2) There are also root vegetables with edible leaves that can be used like a leaf vegetable (onions, potato, carrot, etc).
So that might solve the fast, short-term annual question.
Any thoughts on perennial cover crops? I divided these into
1) Leafy like clover, mint, chives
2) Woody like oregano, thyme