If you don't want to step up to a full monitoring solution such as Nagios you can create your own scripts for monitoring the things that you want to monitor, such as disk space. The following script alerts you when your root partition is almost full:

#!/bin/bash
CURRENT=$(df / | grep / | awk '{ print $5}' | sed 's/%//g')
THRESHOLD=90

if [ "$CURRENT" -gt "$THRESHOLD" ] ; then
    mail -s 'Disk Space Alert' mailid@domainname.com << EOF
Your root partition remaining free space is critically low. Used: $CURRENT%
EOF
fi

The script sends an email when the disk usage rises above the percentage specified by the THRESHOLD varialbe (90% here).

To run it daily, for example, save the script to the file sample.sh in your home directory, change the email to your email, and add the following line at the end of /etc/crontab file:

@daily ~/sample.sh