Additional Image Sizes does exactly what you’d expect, this could possibly be the simplest way to create new image sizes without additional coding within a templates functions.php file. Once installed the plugin adds a new menu below the main ‘Media’ heading in the WordPress dashboard which is where you can name and specify the dimensions for new image sizes. You’ll also be given the option of whether you wish to crop an image to the exact specified dimensions, or whether you just wish to resize the image to the specified dimensions.
Personally I’ve found this perfect for sites where I may need to create a custom sized image for a banner script, yet have already used the standard thumbnail size for featured images on a news article which just gives me that extra bit of flexibility.