League & Tournament Software
There are SO many different ways that people like to run leagues and tournaments that there's simply no "best" software tool for the job. But after playing with several, these are the ones that I've used personally and enjoy. Click the logos to visit the various sites.
This is a spectacular tool for handling ladder leagues, which they call "Shootouts". The reason I love it so much is that it allows you to let people signup on their own, then games aren't scheduled until you're at the venue and have people are actually checked-in. This means that no-shows don't affect you the way a more rigid league structure does.
They offer other random play format, but I'll admit that I haven't seen the utility there. If you enjoy Mix-It-Up or ABCD play at your club, I'd love to hear from you!
Finally, they offer league and tournament software. The costs there are higher than other options, and the UI isn't as refined, so I haven't used them for this. But seriously, the Shootouts are WONDERFUL!!!
Global Pickleball Network
This is a HIGHLY POLISHED site with tons of pickleball features, and it's FREE. Wow! You'll have to advertise elsewhere, though, as you probably won't reach enough people through GPN alone to fill a tournament...unless you do small ones like me, then it's fine. :)
You can run leagues (with their own ratings calculator), but honestly the data entry is cumbersome enough that I much prefer Pickleball Den.
You can run tournaments, and I've been very happy with the round robin tournaments I've run with this. I've not tried elimination brackets, though I'm sure they work smoothly, too.
If you want a very traditional league (fixed partners, playing for a fixed number of weeks) then this is SPECTACULAR! Seriously, the user interface is second to none. I LOVED using it. They aren't pickleball people, though, so the concept of rotating partners is lost on them, but for any traditional league, they're hard to beat! They offer tournaments, too, but I've not tried this, and honestly I feel like a pickleball-specific tournament tool will do a better job (printing score sheets, etc.).