If you haven't yet, go buy yourself an instant read thermometer. You'll thank me later.
This is the one I have, it's about $15 at any kitchen store (Bed Bath & Beyond, Kohl's, Target, etc)
It's got all the temps pre programmed into it, pretty sweet deal. DO NOT leave it in the meat while it's cooking.

Beef And Lamb Internal Temperature Chart
Roasts, Steaks & Chops Rare 120 to 125 degrees F center is bright red, pinkish toward the exterior portion
Medium Rare 130 to 135 degrees F center is very pink, slightly brown toward the exterior portion
Medium 140 to 145 degrees F center is light pink, outer portion is brown
Medium Well 150 to 155 degrees F not pink
Well Done 160 degrees F and above meat is uniformly brown throughout
Ground Meat 160 to 165 degrees F no longer pink but uniformly brown throughout
Poultry (Chicken, Turkey & Duck) 165 degrees F cook until juices run clear, no pink meat
Pork Roasts, Steaks & ChopsMedium 140 to 145 degrees F pale pink center
Well Done 160 degrees F and above meat is uniform in color throughout, no pink meat
Pork ribs, pork shoulders, and beef brisket 160 degrees F and above medium to well done