Is there a mod out there for FO3 which makes all NPCs ignore your crimes so they don't attack you or tell you to piss off?
I can't get into rivet city marketplace even though that guard at the front has already talked to me, well, I can enter but I'm apparently breaking the law and the area is off-limits to me.
Is there mod that will prevent npcs from attacking you or becoming hostile to you when you commit a crime?

The Rivet City Marketplace is only available during business hours. So, if you go there during the evening, the guards will assume that you are a theif. As far as I remember, guards forget crimes after 3 days, so if you wait for 3 x 24 hours, you should be in the clear.

By any chance did you get into Rivet Marketplace at midnight?
You cannot into that market place between 8:00pm-8:am.
As for crimes, after three days they will forget.