I have used eG Enterprise from eG Innovations in a few projects recently. Found it to be very easy to use. Set up is real easy - predefined models made my job very simple. Very deep visibility particularly into the Citrix application suite. The tool has a lot of breadth - it monitors all your typical enterprise applications (web, databases, java, email, etc.). Also can monitor virtual platforms.
I found the reporting capabilities of the tool very useful - particularly when I had to present reports to my customer demonstrating where problems had happened in the past. Overall, recommend this to any consultant or enterprise looking for a simple and elegant monitoring tool.