Although back from the dead, Digi modules are easy to work with standard footprint and AT commands or API support. Cost is low and availability not so good I suppose.

Then there are http://www.synapse-wireless.com same footprint for most of the modules as Digi's and XBee emulator (!) as I can verify with limited compatibility and similar cost, but availability in Europe is a minus.They can be programed in Python (for those who know or want to learn).

I have also tested some of www.dorji.com modem and ISM modules and can recommend them. Especially the DRF7020D27 module that is powerful tranceiver.

Google will show many others too.

Ioannis