Nagios Alternatives

I'm looking at monitoring solutions as we ramp up our datacenter. This is new territory for me and I don't know a lot about what's out there, but Nagios seems to be one of the standard solutions to the monitoring problem. Is there anything else I should seriously be considering? Why?

Edit: So it looks like Nagios has the biggest community and the largest set of available plugins, whereas Zabbix is a more tightly integrated solution that, while extensible, really tries to provide most of the goo for you. There's value to both approaches. The check API for Sensu is really nice.

Thanks everyone! I've got quite a few things to play with now.