One thing that's been bugging me about English recently:
Let's say I stole Joe's gym socks. Then a month later I went to Joe and said "I'm sorry I stole your socks."
Joe's typical response would be "That's ok" (even though he actually doesn't think it's ok that I stole his socks) or "I forgive you" (which implies that he was holding a grudge against me the whole time, and hadn't forgiven me until that point), which he wasn't.
Basically I can't figure out an honest way to respond to "I'm sorry," where the other person actually did something bad that the victim is not really holding a grudge about—at least not without a lengthy diatribe.
Another common response is "No problem"—which doesn't exactly address the issue at hand.
What he wants to say is "I forgive you, but you shouldn't have done that, even though I never really worried about it."
It makes me want to look at other languages for appropriate responses.
Best Answer
"Thanks, no harm done," suggests that you acknowledge the grievance but that it wasn't significant enough to distract you until the apology, along with the recognition that the apology was both due and duly acknowledged, tying up the loose ends nicely. :-)