the Operation Flashpoing Editing Center (
www.ofpec.com) has some nice missions and mods to get you started . Also you can search thru the forums at
www.bistudio.com for releases. Other than that, it's been so long since I've played OFP that I've forgotten most of the www sites I went to. That, and I think a lot of them switched mod making for ArmA when it came out.
They should have links to other OFP fan sites at OFPEC so you can look thru those (although a few are in foreign languages).
One of the better mods I played was CSLA (Czechoslovak Liberation Army). Had a few new weapons/vehicles and a nice campaign.