If you'd like to go for a basic free service, here are a couple of such services:
siteuptime.com (HTTP, POP, SMTP, FTP) and
websitepulse.com (HTTP). Note that Websitepulse also features a free trial where you can try services valued at up to 75$, for two weeks.
I've been using
host-tracker.com and
uppanel.com and I find them quite good. They allow you to check your site quite often, especially considering that they're free.
If you decide to do your uptime monitoring "in house", there is software that you could use. Free software and paid software.
I can't really tell you which software is good or bad, but, as I've always been a fan of open source, I will invite you to first take a look at this free program:
zabbix.sourceforge.net Other software that you could use are:
Netstatus monitor 2.0 - download mirror - free
Nagios.org - free
http://www.rushland.net/projects.htm http://www.ks-soft.net/hostmon.eng/index.htm