Our game, Gravitas: The Arena, is using a custom high-level networking solution built on top of Lidgren.Network. We've had a few lingering networking bugs for awhile now that have gotten buried under other priorities. Today I sat down to try to solve all of them, and luckily enough managed to do so.
Aside from a few minor bugs, there were two remaining major issues that I wanted to address today:
Intermittent, extreme packet loss over LAN
Packets not coming through before a notable delay over the internet when using UDP hole punching (but not Hamachi)
I've been working on a miniature game engine (lovingly called Mingine) as a little side-project lately. It's very barebones right now - pretty much just a platform layer based on SDL with a sprinkle of WinAPI, a mostly-functional graphics API abstraction layer (with an OpenGL backend), some image loading code, input handling...