Gallery 2

• Multiple uploads via JavaScript (can add more file upload boxes)
• Uploads by e-mail, ZIP or multiple image files
• Configurable to run from multiple sites from one install
• Reorder album via JavaScript
• Ratings
• Plugins available
• Can’t protect original sizes of images

Zenphoto

• Multiple uploads via JavaScript (can add more file upload boxes)
• Uploads by ZIP or multiple image files
• Spam filter options (Akismet, CAPTCHA, SpamAssassin, …)
• Reorder album via JavaScript
• AJAX editing of albums and images
• Ratings
• Watermarks
• Can’t protect original sizes of images (except with a watermark)
• RSS feed

Plogger

• No multiple uploads via JavaScript (can’t add more file upload boxes)
• Uploads by ZIP or single image files
• Import from folder (you can FTP all images to a folder and have it import the images from that folder)
• You can protect the original size images
• RSS feed

All of these have a clean interface and design, search, EXIF data, subfolder/album creation, cruft-free URLs (for SEO), and comments (Gallery requires the comments plugin to be enabled).

It comes down to what your needs are, if you want something that can run on multiple sites Gallery 2 is the app to choose. If you have to protect your original size images you’ll want to use Plogger. I personally think Zenphoto has the most to offer and the best design out of the bunch, but they are all very well done. I went through all the ones on OpenSourceCMS and these were the best. Coppermine is popular but it’s too ugly and the admin is kind of confusing.