Actually, "Extending System Center" is a pretty broad title, because that could cover at least an entire chapter in a book (and it does, in System Center Operations Manager 2007 Unleashed!). However, I wanted to mention a new product, Essential Business Server (EBS 2008), available from Microsoft as a Release Candidate beginning May 2008. The EBS product, geared towards the medium-sized market, runs on Windows Server 2008 and uses Essentials in the background. Essentials is part of EBS, but you don't actually see it. This is a very cool example of how Microsoft can use the System Center technology in the background as the management component for other products.  

For information about System Center Essentials, (which is a plural "EssentialS"), see my previous blog posts at http://www.networkworld.com/community/node/23533 and http://www.networkworld.com/community/node/21315.

For a pre-release look at the EBS product, check out my co-author John Joyner's post at http://ops-mgr.spaces.live.com/blog/cns!3D3B8489FCAA9B51!576.entry. The entry also discusses integrating EBS into Microsoft's Remote Operations Manager offering.