The CC2531 is a complete ZigBee / IEEE 802.15.4 transceiver bundled with an 8051 CPU and an USB interface on one chip. You can get these for a few dollars from aliexpress or ebay. Since it has a programming port, you can upload/install a custom firmware which acts as a zigbee concentrator.
Once you have the correct firmware installed, you can use it to replace most if not all of your proprietary ZigBee gateways and use it to control a variety of different devices, such as lamps, temperature sensors, switches, etc.
Depending on your home automation system, you can either use the stick directly with an appropriate adapter or have it plugged into a raspberry pi (or orange pi, etc. ) and use it as remote ZigBee to MQTT gateway which can be placed in a suitable location in order to get a better range.
[2019-01-29] update: new Z-Stack-Firmware with group support and increased stability from Kkoenk available: https://github.com/Koenkk/Z-Stack-firmware/tree/master/coordinator/CC2531