The Web application at could not be found. Verify that you have typed the URL correctly. If the URL should be serving existing content, the system administrator may need to add a new request URL mapping to the intended application.

I was getting this error all the time when I was creating a Windows Form to do some kind of Sharepoint 2010 House keeping.

I found out that everything was caused because I was developing in 32bit instead of 64bit, so to sort it just go to:

1- Visual Studio
2- Right Click into your project and select Properties
3- Select Build
4- On Build go to "platform target" and select Any CPU
5- Compile!

Ah! Don't forget to target your platform to 3.5 otherwise you will not be able to point Microsoft.Sharepoint.dll