any recommendation on network, server and application monitoring software? We have looked at a few different ones (Solarwinds, Nagios, Zenoss...).

Solarwinds seem nice but requires MS server + MSSQL I believe (and is really expensive).
Nagios is a bit geeky.. personally I would choose Zenoss.

Are there any other good ones out there (preferably open source and free)?

Anyone here using zenoss?