We use Systemcenter Operations Manager 2007 R2 (SCOM) and have began to look at its successor SCOM 2012. SCOM uses management packs (MPs) for handling the monitoring logic. Most of these MPs come for free, some are commercial third-party products.
With the Exchange MPs for 2007 / 2010, I firmly believe it sets the standard for Exchange monitoring with its correlation engine. See the Word document in the first link for what it monitors.
Our Exchange 2010 environment runs on VMware vSphere. Her we use Veeam nworks (commercial). Backup Exec is used for backup (free third-party tool). I put a link for the TMG 2010 MP; there are of course MPs for Active Directory, Lync, SharePoint, Forefront
2010 for Exchange and so on. Currently we are investigating MPs for Cisco devices.
By itself, SCOM is an excellent product, leveraging several of Microsoft's own technologies, such as SQL Server with Reporting Services and Active Directory integration. But there is more to it: Moving from HP OpenView (I never remember the new name), it
puts monitoring into the hands of Exchange admins, and there is the same open ever helpful grass root environment around SCOM: MVPs, blogs, TechNet discussion forums.
System Center Operations Manager 2007 R2
http://www.microsoft.com/en-us/server-cloud/system-center/operations-manager.aspx
Exchange Server 2010 Monitoring Management Pack
http://www.microsoft.com/download/en/details.aspx?id=692
Veeam nworks Management Pack for VMware
http://www.veeam.com/vmware-microsoft-esx-monitoring.html
Symantec Backup Exec 2010 R3 (tm) Management Pack for Microsoft System Center Operations Manager (SCOM)
http://www.symantec.com/business/support/index?page=content&id=TECH164049
Microsoft Forefront Threat Management Gateway (TMG) 2010 Management Pack for Operations Manager 2007
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=16814
System Center Operations Manager
http://social.technet.microsoft.com/Forums/en/category/systemcenteroperationsmanager
MCTS: Messaging | MCSE: S+M
