cons2tcod (version 3) is a replacement of the default Windows console, which allows to view roguelikes (and other text mode applications) just like they would look in DOS. Especially useful in Vista and newer, since these systems don't even allow viewing console applications in full screen. However, even on older Windows systems, this utility should be useful, because it uses the correct colors, the correct codepage (if a roguelike uses extended IBM characters, like Crawl, it should look better), and has a possiblity to change fonts.
Currently most important keypresses work. Mouse does not. But it should be enough for most roguelikes.
You can press Alt+Enter to switch full screen or windowed mode. Read (not run) the readme.bat file if you want to change the command line options (e.g., to change fonts or start in windowed mode).

I have tested it (on WinXP) with Dungeon Crawl Stone Soup, ADOM, and DoomRL. DoomRL seems not to work correctly when ran in full screen, but you can first run in windowed mode or the original console and switch to full screen later, then it works.

In case if you don't know how to run your game via cons2tcod: probably you should type something like this: 
cd c:\games\crawl\
crawl
replace "c:\games\crawl\" by the folder containing your game, and "crawl" by the name of the exe file.

Note: cons2tcod is quite a simple program which does one thing and does it well. You can also try NotEye, which also does this particular thing well, but has more useful options (some of which are done well and some not).

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You can contact me at zeno@attnam.com, or at RogueTemple or New Attnam forums.
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Thanks to Slashie for hosting this at RogueTemple!