Browsers
Avant Browser
"Avant is an Internet Explorer wrapper that has a variety of extra features such as a Pop-up Stopper, Additional Mouse Functions, Multi-Window Browsing, and a Built-in Google Search. That's old news though. Avant now is following the path of many apps becoming skinnable. It now joins the ranks of NeoPlanet, Inet-Tabs, and other IE wrappers."
Status: (very) active.
iNet-Tabs
"A new Tab based web browser, designed to be simple and easy to use. It also includes other features like the ability to prevent popups and is completely skinnable."
Status: discontinued, yet available.
Mad Cow (2)
"Mad Cow 2 is a fully skinnable interface for the Internet Explorer engine.The browser is skinnable and so is the toolbar and so is the settings window and finally so are the menus. That's just about everything."
Status: not under development anymore.
Maxthon
Maxthon (formerly MyIE2) is a browser based on IE core(IE5.x or IE 6.0 required), it can open multiple web pages in one window. But it only occupies a little system resource when its has a greatly integrated interface. It also support Plug-Ins & IE Extensions.
Status: (very) active.
Mozilla / Firefox
Better, faster, more secure - browsing with Mozilla / Firefox. Theming stuff is here.
Status: hey, active, ofcourse.
Netscape
"Netscape 6.2, the latest update to the Netscape browser suite, lets you accomplish more online with efficiency in completing tasks, power through more choice and safety with more control. It also supports Mac OS X and Windows XP. 6.2 includes Netscape Navigator, Netscape Mail, Netscape Instant Messenger, Netscape Composer, and Netscape Address Book." At version 8+ now.
Status: apparently still alive. It's still a branded Mozilla though.