PAServerMonitor
http://www.poweradmin.com/servermonitor/
Free and paid versions. Pay to unlock features, clear chart showing which features are included in free, lite, pro, ultra versions.
Easy to setup, powerful, and it just works. I can't recommend it enough.
The hardest thing to get past is the outdated look to their website although it is a very functional website showing clear documentation, pricing, etc.
Edit: Removed link, didn't mean to paste it twice Edit 2: I forgot to mention, they have a free trial so you can see for yourself how insanely fast it is to get basic monitoring setup on your network (ping check, disk space, event log, etc). Easy to build complexity on top of that.
Also, agent free. Supports Windows, Linux, ESXi, etc.
PRTG is a fantastic monitoring dashboard. We are switching to this in January. Ajax web UI and very responsive. Extremely simple to set up and very powerful - http://www.paessler.com/prtg . Free 30 day demo for a good overview. Free Android app which works quite well - Here's the features - http://www.paessler.com/prtg/features
Also used PowerAdmin Server Monitor which was really easy to setup - http://www.poweradmin.com/servermonitor/
http://www.poweradmin.com/servermonitor/
Check it out - free version, incredibly quick and easy to setup, handles the main bases out of the box yet powerful enough to build out complex monitoring and alerting solutions if you want to (I do)
Paid versions are worth it, support is beyond top notch
PRTG converted as well. The web UI is seriously the fastest web ui with all the context menus at your fingertips. Super easy setup, constantly updating the software, Live maps, built clustering solution that is easy to setup and does software propagation to the slave.
A cheap paid solution that works quite well, but few people mention is PA Server Monitor. I used this prior to PRTG, but we needed something a bit more heavy duty. A great little product though, with good support - http://www.poweradmin.com/servermonitor/