Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Ping Meter Gadget - Customizable, graphical, semi-transparent ping (latency) meter.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PingDoctor - PingDoctor is software that is considered to be the most powerful and easiest to use network route analyzer.
Better Uptime - We call you when your website goes down
Fping (open source) - fping is a program to send ICMP echo probes to network hosts, similar to ping, but much better performing when pinging multiple hosts.