Aside from the publishers that already spoke up, here are the following:
Necromancer Games: Their open call for "shorties", regarding their upcoming Bard's Gate product, has been closed. However, if you visit their messageboards, then you'll find a forum that still allows for shorties to be contributed.
Bastion Press: They have an open call for spells and the like for an upcoming product.
Sword & Sorcery Studios: I believe they still have an open call for Relics and Rituals II.