This deal has been around for awhile and might have already been listed by someone else, but I'll post it here anyway since I think it's a particularly hot deal (free)!
US Mobile is giving 100 Days of free unlimited service (50 GB data and unlimited talk & text) for anyone who ports in a number. I think this is a pretty amazing deal, with no strings attached other than porting a number in.
US Mobile is an MVNO on the Verizon network and I've found them to have generally superior coverage and service compared to some other MVNOs on other networks. But of course YMMV depending on your location and network coverage.
Edit: Existing discussion about this deal can be found here.