These are social bookmarking sites, so now you can easily add (or bury) particular posts you like (or don't), using links at the bottom of the respective posts.
For those not familiar with these sites, here's a brief description of each (from the horses' mouths).
"Digg is a user driven social content website. Ok, so what the heck does that mean? Well, everything on Digg is submitted by our community (that would be you). After you submit content, other people read your submission and Digg what they like best. If your story rocks and receives enough Diggs, it is promoted to the front page for the millions of visitors to see.
What can you do as a Digg user? Lots. Every person can digg (help promote), bury (help remove spam), and comment on stories... you can even Digg and bury comments you like or dislike. Digg also allows you to track your friends' activity throughout the site — want to share a video or news story with a friend? Digg it!"
"del.icio.us is a social bookmarking website -- the primary use of del.icio.us is to store your bookmarks online, which allows you to access the same bookmarks from any computer and add bookmarks from anywhere, too. On del.icio.us, you can use tags to organize and remember your bookmarks, which is a much more flexible system than folders.
You can also use del.icio.us to see the interesting links that your friends and other people bookmark, and share links with them in return. You can even browse and search del.icio.us to discover the cool and useful bookmarks that everyone else has saved -- which is made easy with tags."
"A source for what's new and popular on the web -- personalized for you. We want to democratize the traditional model by giving editorial control to the people who use the site, not those who run it. Your votes train a filter, so let reddit know what you liked and disliked, because you'll begin to be recommended links filtered to your tastes. All of the content on reddit is from users who are rewarded for good submissions (and punished for bad ones) by their peers; you decide what appears on your front page and which submissions rise to fame or fall into obscurity."