The CWE Raspberry Pi UART Board is one of the most popular serial communition add-on boards for Rapberry pi 2, as well as Raspberry Pi B+/A+.
Uart board and Pi assembled together
The board uses a FT232R chip as bridge between Raspberry Pi and PC. Compared to cheap chip solutions, such as CP2102 and PL-2303, FT232R provides most relibale performance and great driver support. FT232R is also pre-programmed with unique USB serial number, that implies you can plug into the same PC multiple of FT232R devices.