Directory of Wine Compatible Software

From the WineHQ - About Wine page:

Wine began in 1993 under the initial coordination of Bob Amstadt as a way to support running Windows 3.1 programs on Linux. Very early on, leadership over Wine's development passed to Alexandre Julliard, who has managed the project ever since. Over the years, as the Windows API and applications have evolved to take advantage of new hardware and software, Wine has adapted to support new features, all while being ported to other OSes, becoming more stable, and providing a better user-experience.

From that it sounds like there really is no specific version of Windows that WINE is based off of.

However, if you run wine regedit and then navigate to HKEY_LOCAL_MACHINE > Software > Microsoft > Windows NT:

and select the CurrentVersion key:

you'll see a registry string titled ProductName.

In my case, running Wine 1.6.2, the value for ProductName is Windows XP. Then, looking at the CSDVersion string's value, you'll notice that it says Service Pack 3.

Thus, your safest bet would most likely be that Wine is most similar to Windows XP (Service Pack 3).

EDIT

I've discovered something else in WINE. If you install winetricks and then open it up, then click on Select the default wineprefix:

Then click Run winecfg:

You can change the version of Windows that WINE, I guess, emulates? You can change the displayed Windows version at the very least: