The RadStar RS300 is a continuous radon monitor (by AccuStar Labs) intended for use by home installers and radon mitigators. It records hourly radon measurements in pCi/L for up to 4 days on battery or 10 days with charger attached. Recorded data are accessible via an RS232 serial port. (To simplify access by a PC or phone, I include a USB-to-RS232 adapter, serial cable, and USB cable.)
Like similar devices, the RS300 requires annual recalibration ($150). I just tested it against the Rn levels in my house and crawl space; its measurements were reasonable but low. The device is 4 years old; it is otherwise like new (no abuse; used only in my house). I Frisbeed the original software CD, however, a simple terminal app like PUTTY entirely suffices 
Please see accustarlabs.com for additional details on the device, recalibration, and current software.