Recommendations for MS SQL Server monitoring software or service?
You've got lots of options.  The venders to look at that are SQL specific are:

Quest Software - Spotlight
Red Gate Software - SQL Monitor
Confio - Ignite
Idera - SQLdm
Microsoft - System Center Operations Manager
Microsoft - SQL Server 2005 Performance Dashboard
SentryONE - Performance Advisor
ApexSQL Monitor


They will all do the job nicely, it just really depends on which one works best for you in your shop.  They all have free trials so you can try all of them and make an informed decision.