Digikam is a KDE based photo manager. It's a little more complicated than Shotwell, but can do quite a bit more with your indexed images. Just browsing through the menus it's easy to see that Digikam has far more features and ways to view your photo collection than Shotwell.
Like Shotwell, Digikam uses a database to store metadata for each photo. Digikam can be configured to use SQLite (the default) or MySQL so I spent some time exploring both. MySQL 5.5+ defaults to using InnoDB tables, and scanning my photo directory (41k+ images) ended up taking several hours to finish and had the hard drive churning away the entire time. Switching to MyISAM tables makes the process much faster (a little over an hour) and was a lot quieter. The SQLite option is decidedly quicker, finishing the scan in just under an hour. In either case, the database can get pretty large if you have a big image collection.
Digikam lets you create a hierarchy of tags that you can apply to your images (
didn't notice if Shotwell lets you do thatShotwell 0.11 has this feature). One thing to note when applying tags is that the parent tag is automatically selected when a child tag is applied to an image. Selecting a parent tag automatically selects all the child tags under the parent. Not sure I like that behaviour so it's something to think about when creating your tags.
Digikam synchronizes with nepomuk (KDE's desktop indexer) so things like ratings and comments can go back and forth.
Despite the abundance of features like face and geotagging (haven't tried those out yet) and more image editing options, Digikam isn't difficult to learn. You'll spend a little more time poking around checking out different things but if you're looking for a little more out of your photo management software, Digikam would be worth a look.