If you have enterprise needs, Microsoft's SCOM will monitor availability and performance for SQL Server - and loads of other Microsoft/Wintel products as well. It's competitively priced. Other monitoring solutions are available from the likes of BMC, HP etc
At a budget level, Idera's admin tool kit comes with a SQL Server ping function that will run every x minutes from the system tray - it's USD 295 per install.
Finally, why not try to script one yourself - a dedicated SQL Box, running SQL jobs to query other SQL Server's availability - and using database mail if there's a problem.