By default, OTS sends your hash to three calendar servers. These servers aggregate hashes into a merkle tree over several hours and then commit the root of that tree in a single transaction. So every few hours they make a transaction that commits thousands of timestamps to that block. So the incremental per-timestamp cost is basically zero. If you visit any of the calendar servers, there is a donation address to fund that servers wallet, but the operating cost for these things is really low. I get a ton of utility out of OTS, so I threw $5 worth of bitcoin at a calendar server to chip in. I think that should pay for my use for the next couple years :)

In terms of usecases, I use it to timestamp git commits. One day I’ll have to revoke my gpg key. It’ll be nice to be able to prove which commits were mine before my key got revoked. I also stamp log files and documents so that in the future I can tell if they’ve been tampered with after some date.

Peter Todd commented on twitter that there’s a group using it to timestamp photos being taken in Ukraine to establish “these are at least as old as X” evidence.