We have had great luck, with Ravica Bitsight: http://ravica.com/products/index.php
We use the smallest model, the Bitsight2, we have it at a solar site, monitoring the voltage of a 12v battery bank (which also powers the unit), along with 2 microwave radios and a 12v switch. It works great for this, and they many other sensor types, but it is a bit pricey. It has a nice web gui and users snmp and other forms of notification, and has built in graphing. We used email messages with alerts when certain voltage levels were reached.
-Kyle