Finally, an extension to Mozilla Thunderbird which allows access to webmail! (One of the oft-most requested feature enhancements of the email client. Hooray for extensibility!)
The Web-Mail extension creates a platform which other extensions use to integrate web based email accounts into Mozilla Thunderbird. POP is the only protocol supported, this means Thunderbird can only download emails.
Currently supported via extensions: Hotmail (and MSN), Yahoo and Lycos. A step-by-step setup guide is available.
Update 2005-04-11: Thanks to hints below I've found the option to add webmail accounts in File > New > Account... Any documentation (general or otherwise) on what providers can be accessed by default?
Update 2005-10-04: This extension broke with the Thunderbird 1.0.7 update. (Re)installing 1.0.6 should bring everything back to good. Be advised that older versions of the client lack security updates found in 1.0.7.