> For people that don't want to spend Bitcoin on timestamping it is possible to (ab)use Certificate Transparency logs for the same purpose: https://wiki.mozilla.org/Security/Binary_Transparency

To be clear, OpenTimestamps doesn't require you to spend money to create a timestamp, as it has a set of public calender servers that pool timestamp requests, allowing all OTS users to share the same BTC transaction (which itself is paid out of donations).