Math.NET Symbolics is a basic open source computer algebra library for .Net and Mono. Written in F# but works well in C# as well. Supports .Net Framework 4.5 or higher and .Net Standard 2.0 or higher, on Windows, Linux and Mac.
A callback-based program option parser for C#. See the documenation at http://docs.go-mono.com/?link=N%3aMono.Options and some examples in the code and under the OptionSet topic at http://docs.go-mono.com/?link=T%3aMono.Options.OptionSet
Ookii.CommandLine is a powerful command line parsing library for .Net applications. It allows you to easily define required, optional,
positional and named arguments, parse the command line, and generate usage information.
This is an agile HTML parser that builds a read/write DOM and supports plain XPATH or XSLT (you actually don't HAVE to understand XPATH nor XSLT to use it, don't worry...). It is a .NET code library that allows you to parse "out of the web" HTML files. The parser is very tolerant with "real world"...
CSS Parser written in C#, based on Mozilla Firefox code.
Supports parsing of all modern CSS expressions. Supports Quirks mode. Values are parsed into detailed structures. Detailed error logging. Debugging via SymbolSource.org.