Except in California and Illinois as state law forbids these types of terms in contracts; and as an employee or contractor by law you are sometimes not even allowed to sign theses rights away. [0]

[0] 'Moreover, some states, including California and Illinois, have laws that limit the effectiveness of assignment-of-inventions agreements where the invention was created outside the inventor's employment with the company.' -- http://www.masudafunai.com/showarticle.aspx?Show=5942