My favourite is Sexy Bookmarks which can present sharing options at the top or bottom of ever post and page. You can select which sharing options you’d like to display, and of course it’s free 😉
Another one is Add This and yet another one is Share This. They all work basically the same way, they just have a different look and feel to them. Some themes (like the WooThemes) have this option built in so there’s no need to use a plugin for this.
If you’re on WordPress.com then you can use the integrated option under Settings – Sharing. Drag and drop the services you’d like to use, hit Save Changes and you’re good to go.
You don’t need a Facebook Fanpage or Twitter account yourself to use these sharing options – it’s more meant for people finding your article which they would like to share on their accounts.