Simple Mobile is an MVNO operating over the T-Mobile network. Be aware the $60 "Unlimited" plan is not unlimited (as in TRULY unlimited) and probably the biggest downside is no roaming. If you go where there is no native T-Mobile coverage you will get no service.
As for competitors in the same price structure...not much. AT&T and their MVNO's (H2O Wireless, for example) offer similar plans but not much (if any) data usage included. It really depends on your location and which carrier has the greatest depth of coverage for your needs where you use it the most.