well there are quite a few low gain od's I like when I play blues
ibanez Tubescreamers: I like the nice midrangy ts9 but I like the TS9DX too which has modes with more bass and less gain
boss blues driver: cool gritty "just breaking up" tones
Boston OD series : good pedal when you're on a budget
since I am quite partial to MI audio, I like what I hear from the blues pro. like it more than their blues boy
maxon OD9- the makers of the original ibaneZ tubescreamers
if you wanna go really boutique and are willing to wait, the klon centaur is cool too