The two most-common commercial options are Impinj and ThingMagic readers. Neither are open-source (hardware or firmware) -- they just have APIs. If you go back to the old ThingMagic M5e (rather than newer M6e), there are high-level open source libraries that I wrote:

Documentation: http://wiki.ros.org/hrl_rfid

On GitHub: https://github.com/gt-ros-pkg/hrl/blob/master/hrl_rfid/src/h...

You can buy single-chip readers from AMS (eg. on DigiKey) and build your own reader -- not a light undertaking due to RF PCB layouts, but manageable. I've also heard of people using the USRP (GNU Radio) to build gen2-compliant UHF RFID readers, but I believe they require the PCI-bus variant to adhere to protocol timing standards.

Fantastic! I appreciate the heads-up. The open source ThingMagic drivers would be very handy.

It has crossed my mind to try integrating a single-chip reader but I'm unsure of what the FCC requirements would be for selling it as part of a commercial product. It's a little surprising that there hasn't been a kickstarter type of project to build an open source reader chip and get it FCC spec'ed.

The AMS chip itself is probably FCC compliant, since the output power is so low (10-13dBM, IIRC). The problem arises when you add an external PA to get the full 30dBm output power permitted by the FCC; then you need to re-certify.