OpenSMART, the Open (System|Source) Monitoring And Reporting Tool, is a Open Source System Monitoring Software to monitor problems on your servers and to report some important data about your servers. It is somehow similar to Tivoli or OpenView.
SB EventLog Monitor is monitoring and consolidating Windows EventLogs. Events are collected from server using VBS and WMI or by Windows agent. Events are parsed using PHP and YOUR rules and are stored in MySQL database. Email alerts are supported.
ServerSpy is a VB.Net based application that installs as a service and serves to monitor uptime and availability of other networked servers and web sites.
SWAL is a program that can launch as a service (or standalone) and monitor other services and control other programs. Written in C++ and might have some .NET components (planning on a linux port)