yr-Cerro Tololo. Tells you about the percentage of cloud coverage and their height, although in somewhat sparse time intervals
Skycal The graph labeled "Meteogram" tells you if the sky will be clear (top bar; black and blue is good, white is cloudy), the size of the seeing disk (points and diamonds under the top bar), the temperature (red line), the Moon (yellow line). (Baumer: shouldn't have to do this anymore:) Keep in mind that you have to set the geolocation on the upper right yellow box. Click on whatever the default location is, then type "Cerro Tololo, Chile", and then click on the Tololo location on the map. After that, it will ask you if you want to setup and account and other things. You can do that, or just ignore it and click "OK" at the bottom of the page. After that, the page for Cerro Tololo should appear.