ShareThis has been around for a long time. Once activated, go under Settings::Share This. You can pick from an assortment of button sizes, opt in to Google+, decide where to show your buttons, and whether to do it on pages, posts or both.
Facebook, Twitter & Google+ Social Widgets keeps it simple, and lets your readers “Like,” retweet, or share on Google+ your choice of posts, pages or both. You can add the links to the top or bottom of posts, and also include the links on your home page or archive pages.
Share Buttons by Lockerz / AddToAny works very well. Activate it, then go into Settings::Add to Any to set your preferences. There is a choice of icon sizes, post/page limits, top-or-bottom of post positions.
And there’s also the very simple Google+. There are a lot of configuration options, but basically it does what you see at the top of this post. Readers can click and share with their circles.
Digg Digg lets you put a static or floating, vertical or horizontal, bar of social sharing buttons on your posts, pages, home page or archive pages. Select which ones to show, where to put them, and how big.
You might have to try several, they don’t all work with every theme.