The PicoBT 2 is a Bluetooth 2.1 Expansion which uses the HC-05 BT Module and I2C for communication leaving all other precious I/O pins free. Supports full AT command-set and is of course complete with Piccolino library and examples (on GitHub). This module like the HM-10 can be configured to act as a server or a client, allowing device-to-device networking.
In addition, this expansion adds 5 digital I/O which you can use with simple digitalWrite/digitalRead commands (see library).
Information about the HC-05 including the full AT commands is available below.