Recommendation for free event monitoring software
Polymon from codeplex is ideal for my uses.  I'm monitoring not just server availability, but sql agent jobs success, a host of perfmon data, SQL locking/blocking, db and filesystem freespace, and a whole lot more.

It has a very nice Powershell interface, so any Powershell script you can imagine, it will run on the schedule you specify and then alert/notify/take action as you direct.

It requires a SQL2005 or greater instance to hold its db, but it can monitor any version of SQL.  (Using Powershell, I even have it monitoring CACHE on AIX.)

No cost besides having an available SQL 2005 server to host the db.