I'm looking for a contact or website to turn in a person I believe is part of a huge scam. He's offering 5.5 million dollars :-) It might be money laundering.
The US government maintains a website meant as a portal for all topics related to scams. It has a listing of reporting mechanisms. It's a good resource, since you can read through the reporting section and decide which authorities you should report a specific scam to.
Separately from that, if the scammer has suggested or attempted using any specific company or entity as part of the scam, you should do your best to report the scam attempt to that entity. For instance, if they suggested you open an account at a specific bank or credit union, let that institution know of the attempt. Or, if they suggested you make payments via a specific payment network (i.e. PayPal), or they wrere using Craigslist or eBay or some other website as part of the scam, you should inform those entities. Many times, scams are designed based on what a scammer knows about loopholes or exploits in a specific bank or website; informing the entity they targeted of the scam attempt can help them close those loopholes and/or investigate the attempt as necessary. This can help shut that particular scam model down, which makes things safer for future victims.