I'm sorry if this has been discussed before, but if it has, I still think it will be useful for some for me to post this. This is a site where discussion of getting boxes is in nearly every thread, which makes a thread about this hard to search for.
Anyway, you can get 25 free USPS shipping boxes (per day!) through the USPS site:
http://ebaysupplies.usps.com/
No shipping, delivered right to your door. You have to be an ebay member, but that's about it. I think you can get 25 of each type of box, once per day. Sort of nice considering how pricey they can get when you are sending multiple games to folks.
We could certainly use them for shipping games to each other. Hope this helped someone.