Command line parser library
It parses command lines like the following one for you: drinkmachine.exe /chilled /money:2 /drink:coke /drink:icedtea
It has a lot of options and can even automatically display the correct usage if needed.
I'd vote for its integration within the .NET Frawework.
Update: Stefan Demetz points out that there is a command line parser in Genghis too.