TuniLame

You will be able to configure the color, size, link and link’s target of each button and even add a defaut button for many posts. 
To use it, just take a look on the plugin’s configuration page which contains all the informations and settings for the default button. 
Add the button by using <code>[button color= size= link= target=]text that will be shown as button's value[/button]</code>
Attributes that can be used: 
* color: There is 7 different colors: pink, magenta, green, red, orange, blue and yellow 
* size: There is 4 different sizes: small, medium, large and super 
* link: The link of the button. Example: Your site’s URL http://mysite.com 
* target: The link’s targeted window, can be: blank, new, parent, self or top 
Exemple: <code>[button color=blue size=super link=http://twitter.com/tunilame]Follow me![/button]</code>
Simply add <code>[button]</code> in you posts when you want to use the defaut settings.

CSS3 Buttons

Add coloured CSS3 buttons on your posts using simply a shortcode.

osuthorpe

Ever wanted to add great looking download links on your site? Need to add call to action buttons but don’t want to code? Easy Buttons is the answer. It is the easiest way to add beautiful buttons to your WordPress site. With 6 colors, 3 styles and 2 sizes you can have the perfect look for your site.
Each button is created with only CSS3 making them light weight and easily customized. With a few lines of code in the plugins admin section you can add your own custom buttons.

Easy Shortcode Buttons

Add demo, download and call to action buttons to your site easily with the Easy Buttons Plugin.

CSS3 Buttons vs Easy Shortcode Buttons