SQL Server and Windows offers a whole bunch of monitoring and management stuff for free. Perfmon and server-side traces can be used to keep an eye on your systems. If you have Enterprise, you can start putting the Data Collector to work collecting performance monitoring on your production systems. You can put on all kinds of checks with Policy Management. It's easy enough to schedule specific checks through SQL Agent and set up alerts that will generate emails... You really can build out monitoring for all your servers for effectively nothing.

That said, if you're really interested in something lightweight and inexpensive, you should take a look at Red Gate SQL Response.