DeSmuMe is one of the best emulators for the handheld console Nintendo DS (and Game Boy Advance) that you can find. It lets you use your PC to play the majority of the games in the console's catalog, using many additional features that improve the experience with respect to the original device.
The emulator shows both screens of the console. The one below, which corresponds to the touchscreen in Nintendo DS, can be controlled using the mouse, while the traditional pad can be used with the keyboard as well as with an USB gamepad connected to the computer. This way, you can play all the console's classics, like New Super Mario Bros, Pokemon White/Black, Animal Crossing, and the first games of Professor Layton. You can also use post-processing filters to improve the visual experience.
Among its other features, this emulator also lets you save the game at any time, take screenshots and videos of the games, and use advanced settings to show layers that are useful if you want to rip content from the game.