Opentrackers.org posts open signups. TorrentLeech is at least as good as the public ones, and it has open signups a few times per year.
Otherwise, you can apply/interview for RED and a few others, work your way up through the ranks, and use the invite forums. This is how you get to the really big and really good trackers. But it's also a lot of time and effort, which most people don't want to do.
You might get lucky on the open signups. Every top-tier tracker was once a tiny site with no users. If you join and contribute, they could be the next big thing, and you joined with minimal effort.