I use a Nikon SB-800 and its minimum manual power level is 1/128. That is true for Canon's 580EX II as well. But what if you needed to get even less power than that?
If you use TTL, you can get a lower power threshold, but I know many strobists are loathe to use TTL. The solution is easy - increase your shutter speed to force the flash to go to high speed sync. HSS will reduce the power of your flash by at least 2 stops. With HSS, the 1/128 output effectively becomes as low as 1/512!