Simplenote, Simply Awesome!
I'm a big Evernote user. It's a powerful commercial program that allows you to sync text, photos and documents across multiple devices. Sadly, there's no native Linux client. Also, it's a proprietary software package, and that bums me out.
Simplenote has been an alternative to Evernote for quite some time now. It's created by Automattic, the folks behind WordPress. It's designed to sync only text-based notes, but that's usually all I want anyway. Recently, the developers at Automattic decided to release Simplenote as open source! They also compiled binaries for just about every platform out there, including Linux!
The data is synced into their cloud, so if you're not keen on someone else keeping a copy of your data, Simplenote might not be for you. The convenience of multi-platform synchronization is worth it for me. Since the software itself is open source, a person could look and see exactly what they do with your data as well.
(For a look at the Android-specific version of Simplenote, see this article.)