Instant Username Search - Check out if your username is available
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.
Handle Horse - Monitor when your desired Twitter handle becomes available
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Get Social Domain - Search Domain and Username in the same place
Better Uptime - We call you when your website goes down