The Open Game Libraries is a collection of separately usable C++ libraries for creating games. It provides OpenGL window management, zip file management, OpenAL audio management, a developer console window, input management, bitmapped fonts with Unicode and kerning support, image loading and saving, model loading and importing, and generic classes and objects for game development such as math and text processing.
ENet for C# wraps the C-language ENet networking library. For games and other realtime applications, if you use TCP, old data can hold up newer data, even if it is no longer relevant (old positions, etc.). Your players will perceive any lost packet as a "lag burst". ENet supports multiple in-order streams of data, and allows you to decide on reliability on a per-packet basis. The library is useful for both client-server and peer-to-peer architectures.