In isolation, I've had a few brief stretches where I spent some time doing low level algorithmic stuff. I wrote some sorting/searching stuff by hand once, because I was working on external files, not doing it in memory. There was, at the time, either no library that did exactly what I was trying to do, or I didn't know about it.
More recently I've done some graph theory based stuff (Floyd-Warshall algorithm for "all pairs - shortest paths"), as I was experimenting with some social network analysis stuff that I wanted to do. But even that probably won't go anywhere. As I explore the capabilities of existing off-the-shelf graph database products, I'm finding most - if not all - of what I need.. At one point I thought I might need to roll my own, but it's a burgeoning field and there are more and more OSS options popping up all the time.