Oops, forgot about the widget question:
There are so many things you can use for your social media buttons but personally I like to use a simple text widget (in your dashboard under widgets). I use icons provided by the social media sites I want to list and use html to make the images clickable in the sidebar.
A couple other (easier) options are:
http://wordpress.org/extend/plugins/social-media-page/
http://wordpress.org/extend/plugins/follow-me/
I have not used either of these but they have good reviews.