Microsoft has launched MS-DOS Mobile for Lumia smartphones ..... Says wpcentral

A new OS designed especially for Lumia smartphones. In case you didn't know, MS-DOS was installed on millions of desktops. Well, now you can install it on your phone. "Black and white text has never looked so good," says Tom Messett from Microsoft Lumia Marketing in the launch video. Watch the video to see glimpses of the new/old OS before deciding if you should install it on your phone

The 'internet.exe' command in the same folder opens up Internet Explorer. You'll briefly hear connecting sound before launching the app. There are several other programs that you can check out:

Contacts - Opens the contacts list.
Email [address] - After user types a message and presses the done button in the application bar, the platform's email composer is launched
Maps [search terms] - Launches the map app with search terms
Market - Launches Windows Phone Store
Phone [number] - Launches phone app with given number
Review - Launches review page for this app
Cortana [search terms] - Launches Cortana with the search terms
SMS [number] - Launches SMS composer with the number
Before you start complaining how ridiculous this sounds, go check today's date. It's April Fools Day! Isn't this kind of funny? We don't want to ruin all the fun, so we suggest installing the app and navigating through the command prompts. Watch and re-watch the video for some Easter eggs as well. Hint: there's an interesting game you can play.
http://www.windowsphone.com/s?appid=8573c4a8-b9df-4409-90a6-dad2dc0b6b26
This is how the most popular wpcentral targeted April Fools Day ;)