Firefox only (Windows/Mac/Linux): Firefox launched its Personas theming extension nearly a year ago, but now there's a big, search-able repository that instantly applies themes for previewing. Check out what GetPersonas.com offers and how it works.
As we previously noted, skinning Firefox for a new look doesn't exactly add much productivity to your browsing day, but if you're going to seek out that complete desktop theme feel, GetPersonas makes it seriously simple. As in, mouse over a theme to have it instantly drawn onto your Firefox window, or click it once to have it applied to your browser chrome.
You'll need to install the Personas extension from GetPersonas to get started window-shopping and installing themes. Once you do, you'll get a bemused little fox face in the lower-left corner of your browser. The menu that pops up from clicking the Personas fox automatically updates with the newest and most popular themes, but also remembers the last few themes you grabbed from GetPersonas. As for the site itself, it's arranged and ordered a lot like Mozilla's other add-on offerings:
The Mozilla team's also offered up their own screencast detailing how GetPersonas works:
Share you favourite Personas theme links, or totally expected criticism of how much time this could waste, in the comments.