Another way is to use Linked Servers, set up 2 servers, one is sql monitor which has linked server to all other server and another one is to monitor the monitor.

Set up job to run say every 15 minutes, just try to query each servers (dynamic sql) in master databases and if found the problem log it in temp table for email to DBA.

It work fine for big number of servers, max I use to see and help support the system is 312 servers.

All answers above are correct depend on your situation.

JJ