Great design and great idea. I don't know whether many people still look at this, but I discovered it today and if it was advertized and used more. Some suggestions for Stackathlons: - Quantitive Finance and Economics. This one has suprisingly few, while the disciplines are quite closely related. I tried to add another one, but then it gives probably zero users. In fact, this query gives zero, so it's [tag:status-declined] - Mathematics, MathOverflow, Mathematics Educators (Math Masters) [tag:status-completed] - The Three Sciences: Physics, Chemistry, Biology (Science Masters) [tag:status-completed] - Astronomy and Space Exploration. (Space Fanatics) [tag:status-completed] - Ask Ubuntu, Ask Different, Unix & Linux, Super User (Computer Experts) [tag:status-completed] - without Super User - Stack Overflow, Server Fault, Super User, Meta Stack Exchange (Original Tetralogy). Did you know that Meta SE is older than Super User? Probably the largest tetrathlon. [tag:status-declined] - Information Security and Cryptography. Cryptography is of coure highly relevant to Information Security, since our data can be sent securily because of Cryptography. [tag:status-completed] - Biblical Hermeneutics, Christianity. - Beer, Homebrewing. This one may only give one user. [tag:status-completed] - Science Fiction & Fantasy, Movies & TV. This one will probably give a lot of users, but I don't know what to add to it. Maybe Anima & Manga, but I think it then will collapse to zero. - Travel, Expatriates. Living abroad short and long respectively. Both sites need experts on travel documents, so they are related. Travel.SE also advertizes Expatriates.SE. [tag:status-completed] ---- Added May 2016: - Information Security, Cryptography and Stack Overflow might give a good triathlon, maybe instead of the biathlon Information Security, Cryptography. - Mathematics, MathOverflow, Mathematics Educators, History of Science and Mathematics gives at least one user. It might also be interesting to see how it combines with Cross Validated or Cryptography, leaving out Mathematics Educators. Also related are Computer Science and Theoretical Computer Science. - Some combination of Aviation, Earth Science, Expatriates, Travel and Physics. All five will almost certainly give zero, while there are a few combinations of three and four sites that give at least one. - Bitcoin and Ethereum. More cryptocurrency sites are coming, but how does it combine with Stack Overflow or Cryptography? - Music Practice & Theory and Music Fans - Expression Engine ® Answers and Craft CMS, maybe with Stack Overflow