This plugin by Shivendu Madhava is one of my favorites. Small, simple and adds stylish look to your posts/pages. It is so simple that my review consists of examples only!

Examples:

This is a note

<p class="note">This is a note</p>

This is an alert

<p class="alert">This is an alert</p>

This is a help box

<p class="help">This is a help box</p>

This is important

<p class="important">This is important</p>

This is a tip

<p class="tip">This is a tip</p>

This is linux

<p class="linux">This is linux</p>

This is windows

<p class="windows">This is windows</p>

Very simple. Isn't it?