NLog is a free logging platform for .NET, Silverlight and Windows Phone

NLog can process diagnostic messages emitted from any .NET language (such as C# or Visual Basic), augment them with contextual information (such as date/time, severity, thread, process, environment enviroment), format them according to your preference and send them to one or more targets such as file or database.

.NET, C/C++ and COM interop APIs are supported so that all your application components including legacy modules written in C++/COM can route their log messages through a common engine.
Supported targets include:

• Files – single file or multiple, with automatic file naming and archival
• Event Log – local or remote
• Database – store your logs in databases supported by .NET
• Network – using TCP, UDP, SOAP, MSMQ protocols
• Command-line console – including color coding of messages
• E-mail – you can receive emails whenever application errors occur
• ASP.NET trace
• … and many more

Other key features:

• very easy to configure, both through configuration file and programmatically
• easy-to-use logger pattern known from log4xxx
• advanced routing using buffering, asynchronous logging, load balancing, failover, and more
• cross-platform support: .NET Framework, .NET Compact Framework and Mono (on Windows and Unix)