Firefox only (Windows/Mac/Linux): Mozilla's in-house syncing extension, called Mozilla Weave, updated today with a new, polished UI for setting up and configuring your sync preferences, "major performance improvements", better error handling, and support for syncing Personas, Firefox's on-the-fly theming add-on.
(Click the image for a closer look.)
Comprehensive browser sync is near the top of our Firefox wishlist, and Mozilla Weave is the extension that aims to do that. Weave currently supports syncing your bookmarks, forms, history, passwords, preferences, tabs, and now, Personas. And while this update is nice, it's still short of the pie-in-the-sky sync we're looking for. (When will we get that elusive extension sync, Mozilla?)
That said, Weave is clearly being actively developed, and between the performance updates, addition of Personas sync, and the extra spit-shine (tweak your Weave settings at any time by entering about:weave in the address bar), it's a solid horse to bet on. Weave is a free download, works wherever Firefox does.