Quentin

Allows to define shortcodes for easy replacement. Handy for recurrent copy/paste’s from sites like YouTube or DailyMotion… But not only. You decide! Also allows you not to write un-valid tags that would be stripped out by WordPress (<code><object></code>s for example)
The syntaxt is Shortcodes’ one, but for legacy reasons you can use this syntax : [key:parameter1,parameter2,parameter3…]

Snips

Easy snippet/Shortcode editor plugin for Wordpress

shazdeh

Easily add snippets and custom codes and functions for execution in WordPress environment. These codes will run safely and will not generate fatal errors, so your site won’t break if you make a typo in your code. 🙂
Perfect tool to use as your site’s <a href="http://justintadlock.com/archives/2011/02/02/creating-a-custom-functions-plugin-for-end-users" rel="nofollow ugc">functionality plugin</a>.

SafeCode

Add snippets and custom functions, without the worry to break your site.

Snips vs SafeCode