Some may have already been mentioned
James Earl Jones: First Lieutenant, U.S. Army
During college, he joined the Reserve Officer Training Corps and became a cadet in the Pershing Rifles Drill Team. Although the Korean War was underway, Jones wasn't activated until 1953. He says he was "washed out" of Ranger training and was instead sent to establish a cold weather training unit in Colorado.
Harvey Keitel: U.S. Marine Corps
Like Gene Hackman, he left home at age 16 to join the Marines, ending up in Lebanon with Operation Blue Bat in 1958. In this 2003 interview, he said, "For me the Marine Corps was a spiritual journey. It's not about war. Our duty is to protect those who do not have the means to protect themselves."
Dennis Franz: Airborne Division, U.S. Army
After graduating from college in 1968, Franz was drafted and immediately enlisted in officer's school. He served 11 months with the 82nd and 101st Airborne Divisions in Vietnam.
Rob Riggle: Lieutenant Colonel, U.S. Marine Corps Reserve
This former 'Daily Show' correspondent, also seen in 'The Hangover' and 'The Other Guys,' has served in Liberia, Kosovo, and Afghanistan, earning over 19 medals and ribbons for heroism in combat. He is also a Public Affairs Officer with the United States Marine Corps Reserve.
Clint Eastwood: Swimming Instructor, U.S. Army
Drafted in 1950, during the Korean War. He was stationed at Fort Ord in California, where, thanks to his lifeguard training, he served as a swimming instructor. He saw the most action on leave: In 1951, a bomber he was in crashed in the ocean near Point Reyes. He and the pilot swam three miles to shore.
Rachel Washburn is a former Philadelphia Eagles cheerleader (2007-09) who hung up her pom poms in favor of Army fatigues. She recently finished her second tour in Afghanistan.

There are more, that's just a few I found