Tomboy supports synchronization, export to HTML and pdf, reminders, and todo lists, and formulae in LaTeX.
A criticism to tomboy is that it is implemented in the mono programming language. In Ubuntu Karmic Koala a port from mono to C++ will be available as Gnote. For the Android there is tomdroid, a project, which aims to be compatible and able to sync notes with tomboy.
This screencast shows how it works and what it looks like.
SynchronizationNotes can be synchronized using ssh, webdav, or locally mounted directories (see this blog post for example). However the caveat is that tomboy saves notes all the time even when just looking at them, so you will always have two versions and later synchronization requires user's decision on which note to keep, which can be very tiresome if you have many notes.
Watch out for Snowy, a coming web application designed to power an upcoming Tomboy Online free web service where any tomboy user can viewing, synchronize, edit, and share notes. First support for snowy is included in tomboy version. 0.15.1.