Simple Windows Monitoring with PolyMon
PolyMon is an open source system monitoring solution that can be used to generate email alerts and analyze historical trends of monitor counters and monitor statuses.
It is based on the .NET 2.0 framework and SQL Server 2005.
The most recent version integrates with Visual Studio 2010
I write some software using VB.NET and C# and this is ideal as an add-on, where I can extend it to allow me monitor application specific logs.