WordPress's valuable plugins

If you are using the wordpress to your website's content management system, these plugins maybe use for.
1: WP Shortstat--Allows you to keep an eye on your website statistics without leaving your desktop.
2: Get Custom--allows you to easily retrieve any post’s meta data.(PS: I very like this, easy to use, easy to edit.)
3: WP Cache Inspect--Direct you to easily clear your cache whenever you feel like it.
4: Markdown--Make you easy to create links, bold text, italic text and more - truth me, it's very effortless.
5: WP Pagenavi--make you easy to add simple page number links whenever you have archives that spans across pages.
6: WP Contact Form--This provides you with an instant web-based contact form for your site.

If you have others smart and free plugins, welcome to feedback me. ^_^