SimplePool offers JSON-API for monitoring your rigs. You can open it in browsers, use in your scripts, apps and monitoring systems, like nagios and zabbix.
You miner is 'alive' if it send a share within last 5 minutes.
"alive" - last share sended not more then 5 minutes ago
"last_submit" - time of last submit
"period" - round duration in seconds
"validshares" - number of the shares in the current round
"invalidshares" - number of rejected shares in the current round
"hashrate" - average speed in the current raund
"hashratelasthour" - the average speed in the last hour
"hashratelast" - the average speed for the last 10 sent shares