Supermium: up-to-date Chromium for Windows XP, Vista, and 7
Modern browsers don't really support older versions of Windows anymore, so anyone running Windows XP, 2003, Vista, and even Windows 7 and 8 are losing access to secure and capable browsers. While running those older versions of Windows on production machines isn't exactly advised, they're still great fun as retrocomputing platforms and to keep older Windows games accessible using period-correct hardware.
As such, there's some awesome news: there is now a fully up-to-date variant of Chromium for these older versions of Windows called Supermium. It tracks current Chromium, supports extensions, sandboxing, Aero Glass, Google Sync, and even Widevine on Windows 7 and higher. Micheal MJD just published a video showing Supermium in action in case you're curious.
You'll need at least Windows XP SP3 and an Intel Pentium 4 with SSE2 in order to run it, and Windows 2000 support is in the works, too.