Adding a vote for the Olympic (or Olympia, I can never remember which it is) Diner at 49th & 8th. I used to live nearby and became a "regular" there; since I moved uptown I still go back on occasion and a few of the staff remember me. Very friendly people.
Edison Cafe is also good for a diner-type place.
Blockheads Burritos in the Worldwide Plaza (49th/50th bet 8th & 9th) is very good, and they have $3 margaritas!
For Italian I like Daniela Trattoria, which someone mentioned below. It's on 8th bet 45th & 46th.
If you like Thai, there is a place on 9th Ave around 47th-48th called Tiny Thai that is very good.
My only other advice is to avoid the touristy places right in Time Square. (Hard Rock, Bubba Gump, Olive Garden, etc). They are all overpriced and usually crowded, even if the food is decent.
Updated On: 5/21/07 at 02:56 PM