Nagios saves me more sleep than I care to admit, but configuring its email alerts if you build from source requires a little more finesse. Specifically, you need postfix (likely you already have) and heirloom-mailx.
On Debian (and probably Ubuntu, but check first):
apt-get install heirloom-mailx
cd /usr/ports/mail/heirloom-mailx make install clean
And NetBSD’s pkgsrc:
cd /usr/pkg/mail/heirloom-mailx make install clean
The BSDs also have binary versions in pkgng/pkgin, ports and pkgsrc. That’s a lot of package managers.