Not all of us work at companies big enough to justify the expense/time it takes to implement something like MS Mom, Altiris, etc. For years I've recommended Servers Alive as a reasonably priced (about $200) solution that lets you do basic fault monitoring. It can alert on low disk space, check that a web page is running, login to a SQL Server (which isn't the same as saying the service is running!), and notify you via email, pager, or instant message when a problem is detected. You can monitor up to 10 items for free, after that you have to pay. Most of us in the DBA world use SQL Agent to do a lot of our monitoring/alerting, but on rare occasion SQL Agent can fail. It's nice to have a secondary level of monitoring running from a different machine.
There are probably similar products out there, it's one of those "I could write that!" type apps, and I stopped looking when I found something that met my needs.