I use HC-110 dilution H some times, it depends a little on the initial development time, actually I often use H when pushing (not sure if it's any better than B though ) I've never used any less than 7,5ml HC-110 though, I simply make up more, so I need more syrup.

In Rodinal, I once did a 1:200 stand and I think I was down to around 5ml Rodinal. Worked ok, but I wouldn't use such a small amount of developer on any critical work.

Caffenol is a pretty good single-shot developer (cheap too), takes just a few minutes to mix it up, and the ingredients keep very well.

TMax 100 in caffenol is very very nice actually.

As you get into higher iso films, you probably need to add some Potassium Bromide to reduce fogging. (the caffenol blog has a lot of data on various film trials).

I use HC-110 and Rodinal, although I do have stuff like XTol and other developers hidden somewhere. (I shoot pretty low volumes, so I need the developer to keep and keep and keep ^^ )