I love to recycle leftovers.  I use leftover roast chicken, or boneless breasts in cassaroles or to make quesidillas (sp?).  Roast beef is a little harder to "recycle" for some reason.  I have used leftover pot roast and gravy to make a pot pie.  I cut up the meat, mix in the veggies and gravy (maybe add a few more frozen veggies like peas or corn) and buy some refrigerated pie crusts.  My family isn't too crazy about gravy so I don't do this very often.  Most of my beef "recycling" involves gravy.     I'm going to use leftovers from yesterdays roast beef to make stir fry - never tried this before, but we like stir fry.  Does anyone have any favorite ways to use up leftovers? 

I will do open sandwiches with left over RB and gravy. Leftover chicken is either soup, chicken salad or cut up on top of a green salad or ceasar salad. Leftover steak is used like this. Saute onions and mushrooms, add the sliced steak top with mozzarella cheese and serve on hard roll. YUMMY.

Instead of stir fry for beef you could make fajitas? I don't know, I don't find too many leftovers exciting, but it's something that needs to be done.
Diney

I do the same thing Barb does, and sometimes I make veggie soup with it. 

Have you ever made beef noodle soup?  It is delicious.  Just take your left over roast beef, onions and sliced carrots and cook in a base of beef broth, then season to taste just like you would chicken soup, and then add egg noodles.  Yummy!
You could also do a variation w/crushed tomatoes, onions and cabbage and use macaroni noodles making it like a goulash soup. 
I like soup. 

Growing up, Friday was always left over-night. I would often make the beef into a goulash type dish. I would boil the beef in water with seasonings, whatever vegetables we had leftover. I would even cut everything up and make a fried hotdish. Where everything went into a large frying pan or electric skillet and then as everything was nice and tender I would add a thickened beef sauce over it. Everyone liked it.

Chicken, I usually make into homemade soup. I even make the noodles from scratch. I is so much better than the canned stuff.

I just thought of this when I saw Ginny's shepherd's pie suggestion. Chop the leftover roast beef and saute with onion, garlic, celery, carrots and green peas. Add a can of cream of mushroom soup and season and pour into baking dish and top with a pkg of cornbread mix or 2 depending on the size of the baking dish. Bake till bubbly. I do something like this with a can of beef stew mix and cornbread mix when I am strapped for time. It is so easy and quick.