It is mainstream now that it is integrated into Java, C#, etc.
It was considered esoteric 20 years ago. Haskell initially had the slogan "avoid success at all costs".
SQL, HTML, CSS were probably esoteric at some point.
Perhaps a more interesting example is the MLs, which have been around far longer than Haskell and seem to be gaining popularity for implementating and transforming other languages (that was their original purpose, after all!).
Scheme's also managed to maintain a minor presence since the 70s, especially in education and as an extension/configuration language.
HTML was regarded as too mundane to be interesting when it first came out - a lot of people working on hypertext at the time (early 90's) regarded it as a simplistic toy.