There's a list of the four calendars on https://opentimestamps.org/; each calendar has a website showing you stats.

The most frequent transactions at the moment are done by https://alice.btc.calendar.opentimestamps.org/, on average once every 5 hours at a cost of 20,000sats/week. 

 Thanks. Open timestamps is a fascinating project. 
I noticed that the transactions are using Segwit. Would there be any significant savings by switching to Taproot?