Dotfuscator

Dotfuscator is software used for obfuscation of .NET assemblies. Among its abilities are mangling of identifier names into unreadable characters, but also changing control flow into being more unreadable at the IL-level for a human and other relatively advanced changes.

Note that name mangling interferes with reflection and whenever it is an issue, identifiers that are used with e.g. Windows Forms or WPF data binding have to be manually excluded.

Control flow changes are designed in such a way that the JIT compiler is able to unravel them and produce identical code. They're not known to introduce performance issues.

Other possible change is watermarking each executable/DLL to make it easy for the copyright holder to harass the copy owner through legal means in case it spreads through file sharing.

The "free" edition available to Visual Studio owners doesn't do much else than name mangling.