I see many blogs that under each blog post, it shows related posts and there are also links to add the blog post to social bookmarking sites. Would someone know, are these plugins or some sort of code?
If they are word press they should be plugins for most and some codes are offered to just put social bookmarks. You can just add code for some social bookmark sites without a plugin also.
If you would like to have social boomarking sites buttons like what Wayne Liew Dot Com has, give obsocialbookmarker a try, a plugin that supports over 150 sites which allows you to pick which to show.