2023 UPDATE: While this plugin may still technically work, I no longer recommend its use. Because it has not been updated in a number of years, it potentially has security vulnerabilities. Instead, I have been using Crop Thumbnails by Volkmar Kantor, which works in a similar manner and is being maintained by its developer.
Read on for my original 2015 post:
We use a lot of WordPress here at Castle Builder. Over the years, we’ve tested, installed, used and removed what seems like an endless number of plugins. When I find a good one that solves a specific problem, I’m happy to share!
WordPress has a built-in “Edit Image” feature that allows you to flip, resize or rotate your image. You can even apply your changes to all versions of the image or just the thumbnail. Great!
However, if you want to use custom thumbnail sizes in your theme—or have multiple “thumbnails” for different purposes using the same image—your editing options are more limited. The Post Thumbnail Editor is very useful in these cases, allowing you to crop the same image differently for each size, including your custom sizes.
Learn more or download on the plugin’s page.
This is for WordPress the software (downloadable at wordpress.org), not the hosted solution available at WordPress.com.