Pete Mall

This plugins enables you to add the official <a href="http://blog.twitter.com/2010/08/pushing-our-tweet-button.html" rel="nofollow ugc">Tweet Button</a> from Twitter. It lets your users share links directly from the page they’re on. When they click on the Tweet Button, a Tweet box will appear — pre-populated with a shortened link that points to the item that they’re sharing.

WP Tweet

Adds the official Tweet Button from Twitter.

Skysa

Promote your Twitter account at the same time while driving traffic to your website. Customize the default Tweet, set what URL to share in the tweet and set the option to show or hide the tweet count next to the tweet button. By allowing your site visitors to tweet about your site on their twitter accounts, it will help get the word out about your site and drive targeted traffic from twitter users who see the tweets directly to your site.
http://www.youtube.com/watch?v=Zj1uGXX2xrk&hd=1 
<a href="https://wordpress.org/extend/plugins/tags/skysa-apps" rel="ugc">More Skysa App plugins</a> -|||- <a href="https://wordpress.org/extend/plugins/skysa-official/" rel="ugc">Skysa App Bar Integration plugin</a>

Skysa Tweet App

Let people share content on Twitter (a Tweet) without having to leave your web page. This app displays a Tweet button at the bottom of your site.

WP Tweet vs Skysa Tweet App