My standard reply is the Old Benoni, 1.d4 c5. It's not commonly seen at the top levels, but at our levels it should be fine.
EDIT: I should stipulate that it hasn't been used much at the top levels *recently* by any top-teir players. It has been used a lot by numerous GMs, especially in the pre-computer era.
I'm looking for a response to 1.d4 but it needs to involve a pawn advance by black... any suggestions??