hamsterdb Embedded Storage is an embedded database engine for C/C++, including wrappers for Java, .NET, and Erlang. It supports in-memory databases, very fast database cursors, "record number" databases, ACID transactions, and duplicate keys. hamsterdb is very fast and highly configurable. It compiles and runs on Unix platforms, Linux, Microsoft Windows, and embedded platforms.
Zyan Communication Framework simplifies development of distributed applications. It works on Windows, Linux, Mac OS, and Android platforms via .NET or Mono runtimes. It features LINQ support for three-tier applications, full duplex TCP support, traffic, compression, and encryption, transparent support for events and EBC (event-based components), and Windows and LDAP authentication support.
NetIRC2 is an easy-to-use .NET IRC client library. It supports all major features relevant to making a chat client or bot. It has full support for synchronization contexts. You can use it from a Windows Forms or WPF GUI thread and chat events will be automatically dispatched, allowing you to completely ignore multithreading. It uses byte arrays internally, so mixed encodings can be handled. The IrcClient class can even be used as a component in the Forms Designer. Because it speeds up connect times with some IRC servers, an Ident server is included as well.
CryptSharp provides a number of password crypt algorithms: BCrypt, MD5 (and Apache's htpasswd variant), PHPass (WordPress, phpBB, Drupal), SHA256, SHA512, and Traditional and Extended DES. It also includes Blowfish, SCrypt, and PBKDF2 for any HMAC (.NET's built-in PBKDF2 implementation supports only SHA-1). If you are looking to store passwords, odds are CryptSharp will have the algorithm you want.
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.
Asbru Web Content Management is an easy-to-use and inexpensive Web content management system. It runs on most major Web platform operating systems, databases, Web servers, and scripting languages. It is available in three editions: Personal (for individuals), Professional (for organizations), and Hosting (for Web hosting service providers).