This app was mentioned in 2 comments, with an average of 2.50 upvotes
Windows is quite covered, so I'll chip in something else
Android app: versus
https://play.google.com/store/apps/details?id=de.versus.android.start.full
Not free, but very cheap. I've been using it for ages, simple, works great.
I can run a tournament of my phone anywhere anytime
I just downloaded this simple little cheap app to make tourneys. Could be useful for people.
https://play.google.com/store/apps/details?id=de.versus.android.start.full