SiteUp monitors your web sites and warns you when any become unreachable (e.g. when a server crashes). Because it checks page content it can even detect when the server is OK, but a site or page is unavailable.
There is no limit to the number of sites that can be monitored, including secure sites, and the checking frequency can be anywhere between once a minute and once a day. When a site goes down the system tray icon will change to an animated alarm and optionally you are displayed a warning message. When a site becomes available again you can view a log to determine how long the site has been down.
Pentium 100, 1 MB HDD, 128 MB RAM