I have been accessing a text version of Slate on my Kindle from the following site:
http://www.angel.net/~nic/slate.cgi
But I don't know how to create a recipe in calibre for downloading, say, all posts on a particular day.
Another site I love is Arts and Letters Daily,
http://www.aldaily.com/ which usually provides links to a few interesting articles everyday. It will be great if one could download these articles once a day in an ebook form.