The WICHE Western Undergraduate Exchange program is extremely broad-based and covers more than 135 public colleges and universities spread across Alaska, California, Oregon, Washington, Hawaii, Nevada, Idaho, Montana, Arizona, New Mexico, Colorado, Wyoming, Utah, North Dakota and South Dakota. You pay a fixed 150 percent of in-state tuition.
WICHE - Student Exchange Programs
Some universities have restrictions on majors allowed, or on the number of WUE students accepted, but in general, it's the broadest "academic common market"-type system in the country.