I recently joined and not having a Synchronet BBS I had to make my own client to work with their service. As promised I am also releasing the source.
It is written in C# against .NET 2.0 and *might* work in mono on linux. You will have to test.
Update: I made a small change to the dropped carrier handling and disconnections. The door should now handle dropped carrier and exit correctly. Let me know if you have issues.
Update: Removed Github links.