DeSmuME is a free cross platform Nintendo DS emulator capable of playing ROMs and homebrew software for PC and MAC.
Not only DeSmuME emulates all the functions of Nintendo DS like buttons, touchscreen interface, and even the Wi-Fi connection, it’ll let you play your ROMs and homebrew apps. You can also take screenshots, create movies from your gameplay, load cheat codes, and save your game state. That allows you to save any game at any time, even if the game doesn’t support profiles or saved games.
Check out NDS Homebrew for a variety of homebrew games and applications.