Wavetrend has various hardware available including tags suitable for mounting on metal, plastic, personnel, and keyfobs with buttons. Tags also support RSSI such that it is possible to determine approximately how close a tag is to a reader.
On the software side the serial data from the RX300 could just be transparently forwarded to a server for processing, however if you have only a small number of tags, and are looking to trigger events on other nodes without depending on an external server processing the data in the middle the better option is to parse the tag data on the Synapse Engine itself.
See below for sample code to parse data from a Wavetrend RX300 reader and send logEvent RPCs to Portal (event log code can be replaced by your own code to e.g. trigger events depending on the tag ID or RSSI). Python code should also be easily adaptable for use on non Synapse hardware.
[Download: Wavetrend L-RX300 Parser]
Update 2011-05-20: Suppliers of WaveTrend hardware include: