T-Mobile in the US used to discount their post-paid plans when you brought your own phone. They stopped doing that but started offering some pretty great prepaid plans:
I'm on the $30/mo "walmart" prepaid plan, but I'm considering switching back to post paid because you can't use any data roaming at all on T-Mobile prepaid.
http://prepaid-phones.t-mobile.com/monthly-4g-plans
I am on the $30/mo unlimited text, 5GB of HSPA+ data and 100 voice minutes plan and it's great. I reduced my monthly bill by 60%.