I've used a couple options before to check on my temps.
Many brands of UPS's have some optional sensors that are either internal or can plug into them. APC is pretty common and has some software that will send you traps or can even page you when things occur. This is probably the cheapest option if you already have a UPS that has a network connection and sensor ports.
The other option would be the standalone sensor. The only system that i have worked with is netbotz. They were previously a seperate company but are now under the APC umbrella, they have a whole range of sensors that will connect into your network and can send out data by snmp. There is a demo on the front page of their site that could give you a good idea of how their system looks.
I don't know if you can buy these in the Sates http://www.climate-monitor.co.uk/products/index.html but they are very good and can have other monitors on them apart from temperature. They're not that expensive either.
We use NetBotz appliances in all of our server rooms. I have nothing but great things to say about them.
We use a custom MIB poller in Orion for our HP servers. The poller MIB OID is: 22.214.171.124.126.96.36.199.188.8.131.52.4 which will deliver the Compaq Health Temperture in Celsius (cpqHeTemperatureCelsius). We then add this MIB poller to certain servers top - middle - bottom in a certain rack. We've also created an alarm to fire if the temperature gets to a predefined threshold.
Hope this is helpful.