I've been playing with git, the new hotness.
One thing I've been puzzled by is the packs; they seem to be almost undocumented. Git stores objects, at request and periodically, in compressed form, in packs. (It also looks like it packs before doing transfers among clones.) These packs are indexed, for better access, but it's a standard, space-time tradeoff.
What's explained in even less detail is the delta chains. Some files in the pack are stored as deltas against other files. These deltas aren't history-based, they're just done by-guess-and-by-golly, and how compressed the pack is depends on how much time you give git to generate them.
This all sounds cool, but I wish I could find more documentation. If you see any, let me know.