[EnderShadow8]

Treap: https://en.wikipedia.org/wiki/Treap

It's like a Red-Black tree in use case, but much faster to implement, which is good for competitive programming. The average case complexity is the same for all operations, but there's an unlikely degeneration to worst-case linked-list behaviour.

Lazy Propagation Segment Tree: https://cp-algorithms.com/data_structures/segment_tree.html

Like a segment tree in that it supports range queries in logarithmic time, but supports range updates in log time as well.

I know a few more fun ones that I occasionally use in contests, but I've never touched otherwise.

[cassepipe]

Speaking of ease of implementation, I just discovered AA trees. They are probably not "obscure" but I think they are worthy of more fame because they perform jsut as well as red-black trees and are easier to implement. Finding clear comprehensive documentation about them was not easy though, so here is for you, the best I could find : https://www.cs.umd.edu/class/fall2019/cmsc420-0201/Lects/lec...

And the, unhelpful to me, orginal paper : https://user.it.uu.se/~arnea/ps/simp.pdf

[kgr]

The Wikipedia article on AA-Trees is good: https://en.wikipedia.org/wiki/AA_tree

[_benedict]

The nicest thing about treaps is how easy union/intersection/disjunction are.