I don't know, what about Performance Monitor and Profiler? They're free and actually do a pretty good job of monitoring the system. You can set up alerts within SQL Agent to respond to Perfmon counters as well as TSQL statements. You can set up Operators, Alerts, and completely get an automated monitoring system in place with the tools that Microsoft supplies. If you're working with 2008 you should also take a look at Policy Based Monitoring for the checks it provides. Also, with 2008 you can get the Data Collector working on gathering performance metrics for you. There all kinds of possibilities before you start looking at third party tools. That said, I like Confio's Ignite Free as well.
Lepide has also a good solution for SQL Server Monitoring Lepide SQL Storage Manager Analyze growth rate of Databases, File groups, Files, and Tables to forecast their future size. Eliminate need of continuously tracking disk size and communicate well in advance future disk requirements. [
You could have a look at a series of articles by [David Bird on SQLServerCentral], where he takes you through the steps of rolling your own SQL Server monitoring toolset using SSIS and SSRS. I've implemented this and expanded upon it several times, and it got me learning SSIS & SSRS... :