OpsMgr’s extensibility is one of the most interesting aspects of the product.  I recommend you check out the following extensions, which I seriously considered but which didn’t make it to my top five list:

Authoring Console: Operations Manager 2007 management-pack authoring console.
CrossPlat (aka Operations Manager 2007 Cross Platform Extensions Beta): Provides cross-platform support for monitoring UNIX systems in OpsMgr (currently in beta).
Eventid.net: Performs a web lookup on eventid.net for events in OpsMgr. (In System Center Operations Manager 2007 Unleashed, by Kerrie Meyler, Cameron Fuller, et al.)
Mass resolve alerts: Resolves alerts that haven't been modified in the time specified. (In System Center Operations Manager 2007 Unleashed.)
Multi-host ping: Custom management pack that monitors systems via a ping test in OpsMgr (includes views and collects ping-response-time information).
Process Monitoring: Alerts if the number of processes isn't running within an acceptable range.
ProxyCfg: Enables and disables the act-as-a-proxy configuration for agents.
Scheduled Tasks: Monitoring for scheduled tasks.
Security Top Alerts Management Pack (STAMP): Alerting for key Windows Server security events.
Service Level Dashboard: Tracking, managing, and reporting line-of-business application service-level compliance.
Silect Software MP Studio: Management pack testing, tuning, auditing, and lifecycle management.
System Center Capacity Planner 2007: Infrastructure planning for several products including OpsMgr.