QTTabBar does more than just add tabs, though. It also adds a ton of other options that make Windows Explorer a bit more powerful. For example, you can copy the path to a folder just by double-clicking on its tab, or by clicking the copy path button in QTTabBar's extra toolbar. You can also add a ton of different custom keyboard shortcuts to Explorer, which is great. If a full Explorer replacement like previously mentioned Total Commander isn't for you, QTTabBar will give you a bit more power in the Explorer you already have.
We've actually talked about QTTabBar before, but it's come a long way since then. Not only has it added a ton of extra options over the past few years, but there is a shiny new 2.0 version, which seriously increases stability. If you tried it in the past and were unhappy with it, version 2 is sure to give your Windows Explorer a new lease on life.
Hit the link below to check it out, and be sure to go through its Options page, as it changes a few common Explorer shortcuts you may be used to (double clicking on a folder now goes one folder up, for example; you can change this on the third "Tab & Window" section of the options).
Update: If you don't like the new version, there's also a fork of the project that you may find a bit more to your liking.
QTTabBar is a free download for Windows only.