Any tag with a battery.
Active RFID is currently built on proprietary standards and one system doesn't work with any other. Most commonly, these are found in RTLS systems (Real Time Location) and, the theory is, you can see all your assets in real time.
We are currently working on an RTLS solution. Once it's ready, we'll be able to make some comments about how cool it is. For now, expect RTLS to cost you $100,000+ and $75/tag.