We've had the best experience with Estimote's proximity beacons so far, but Robin works with any BLE beacon using the iBeacon spec.
We also like the enterprise-grade Aruba beacons, which unlock indoor wayfinding and campus maps.
Beacons range in price from $5 to $90 apiece, and can run on battery or AC power. USB beacons are the easiest to maintain, while battery versions come with mobility. A good battery-powered beacon will last about two years.