I found out the other day that zstd supports dictionary compression [0]. It's a pretty neat concept: you can "train" a dictionary on your corpus of text, and use this dictionary to compress subsequent documents. This is really useful if you have a lot of small documents with a similar distribution of characters. You only have to transmit the dictionary once over the wire once, and then clients can get great compression ratios after that.
For example, suppose you have a lot of chess PGNs you'd like to store [1]. If you compress each PGN separately, you won't get nearly as high of a compression ratio as if you stored them all in the same file and compressed that file. But storing games individually is more convenient. Here are the numbers:
Original PGN file: 780 bytes
Default zstd compression: 508 bytes (35% decrease)
With dictionary: 376 bytes (52% decrease)
I'm thinking about ways to compress at an even higher ratio. Chess PGNs are a great candidate for this task because they have a huge amount of redundant and repeated text.