Easy to delete order in backend through the Action dropdown box on the native Order Manager.

Installation:

1. Install the extension through Magento Connect.

2. Go to System > Cache Management > Click button Flush Magento Cache.

3. Logout out and re-login.

4. If it still not works, delete the file downloader/pearlib/pear.ini and do following step 1 to 3 again.

Usage:

In the backend, Sales menu > Orders, tick on orders you want to delete, select Action Delete at the top right corner.