In most U.S. states (even WA) [Law here](http://apps.leg.wa.gov/rcw/default.aspx?cite=49.44.140). Employers often still make you sign something saying that they own your soul and blah blah but they aren't enforceable.

Now, in some states (WA but not CA or NY) your employer can fire you for moonlighting if they want to but that's about it. You also have to watch out if you signed a non-compete and are arguably competing with your employer.