[VeritasKhan]

If a stranger apologizes for something by saying "Sumimasen" how can you let them know that it's really no problem. For example, maybe they bumped into you, or spilled something on you, etc.

I believe I've heard you can say:
いいえ - (iie) - No. (which I suppose is a shortened version of "It's no problem" or "It's no trouble" or "No, it's not even worth apologizing for")

Would it sound strange to say:
いいです　 (ii desu) - It's good. It's ok. It's all right.
いいですよ　 (ii desu yo)
問題じゃありません (Mondai ja arimasen) - It's not a problem.