Many programmers, I expect, have a favorite obscure language or two they'd like to see in wider use. Haskell has quite a following, though it sees relatively little use; the same can be said for most pure functional languages. Prolog seemed like a neat idea when I first read about it, but I've never heard of anyone using it for something serious (caveat: there are lots of things I've never heard of).
My own favorite underused language is SQL. Although most programmers have at least a passing familiarity with SQL, and many use it daily, few seem to achieve real SQL fluency. This is unfortunate; SQL databases are powerful and ubiquitous tools, and even the least among them can generally manage a great deal more than the fairly simple uses to which they are commonly put.
I recently had the opportunity to trot out this curmudgeonly opinion of mine at the Utah Open Source Conference. Now in its third year, this annual conference continues to surprise me. Utah is home to an extraordinarily vibrant open source community, and it shows, in the attendance (over 400 expected), the number of presentations (92 by my count of the schedule, perhaps minus a few that aren't really presentations, such as the Geek Dinner), and the overall quality and diversity of information presented.
Conference presentations were all recorded, and will eventually be available from the UTOSC website. Most speakers, in the meantime, have made their slides available. Mine are here.