One of my favorite black and white developers, and also one of the oldest developers on the market.

Rodinal is pretty much the only developer in popular use based around the chemical paraminophenol hydrochloride, which is a chemical cousin of asprin in a fairly caustic activator solution. The result is a developer that can be diluted quite heavily, so a single bottle of developer will last quite a long time. It's fairly insensitive to temperature and tends not to cause bromide drag like other developers.

Rodinal has absolutely no grain-minimization effects. In fact, it seems to have infectious development like a line-art developer from making printing plates. Most other developers contain some level of solvents to prevent this effect. It's got substantial edge-effects, which means that you tend to get results that look tack-sharp (even when they aren't).

You can mix up an acceptable substitute for it with crushed Asprin pills, but it's one of those products that managed to survive the failure of AgfaPhoto.