You can use Yahoo! Weather in either Fahrenheit or Celsius (Centigrade.) To toggle between Fahrenheit and Celsius on city pages, click the linked C or F in the upper right corner of the Today section.
If you wish to switch from Fahrenheit to Celsius as the default in the area you are in, the site does not offer that feature. However, here is a possible workaround.
Example: Let's say you are originally from the UK and now live in Boston. When you open Yahoo! Weather, you want to always see Boston weather information in Celsuis. Here is how to do that:
(If you wish to have Fahrenheit as your default, then follow the same process, but go to http://www.yahoo.com as the original site.)