depends on the bolt pattern.
for shimano (and nearly everyone else)
130mm (normal roadie cranks) won't go with a middle lower than 38t
110mm (compact roadie) will go down to 34t, and take any old 5-arm MTB rings (may require some fileing to get the lugs to fit, but the BCD is the same).
For campy, i've no idea
I think there's some touring and CX chainrings (and chainsets if you look) that come with senisble off road gears, well if you considder 48t sensible. I'm running a 48t on my main road bike ATM, because unless you'r pro 52t is pointless, I just cant spin it fast enough to get any real use from it!