b-trees. ugh. i wrote code around b-trees in my data structures class in college. but to my surprise, when i got out, pretty much no one in the "real world" (hiring for entry-level positions) use them. they mostly use things that we didn't ever actually play around with -- stuff like hash tables.
B-trees (and related data structures) are pervasive in the real world. Pretty much any application that requires high speed access from disk uses them. Think about filesystems, databases, etc... They're hugely useful.
But they're not useful for the same purpose as a hash table, which is also a pervasive data structure, but for a different problem area: constant time access to in-memory data.