To name a few, product - company

SQL Monitor - Red-Gate

Idera DM - Idera

Spotlight - Quest\Dell

Foglight - Quest\Dell

SQL Sentry - SQL Sentry

Solarwinds have started doing an application monitoring module which connects to a ODBC database.

Other people will probably chip in with more

My advice would be to get a copy of each (Foglight will be tricky, best off speaking to Quest\Dell as it is a complex install so they usually get a technician to do it) play with them and see which one you like best.

In the past I have used Idera and Spotlight, they did the trick for us.

But it would all come down to your personal preference, what your looking for and what suits you best.