It all really depends what you mean by "substitute". Other similar RPGs, different RPGs, or computer games?
Some similar RPGs include: 'Pathfinder' (relatively new, by Paizo Games) and 'Castles and Crusades' (older than Pathfinder but still really new, by Troll Lord Games) are both games based off of first edition and 3.5 edition D&D (repsectively).
There is a whole slew of games that work similarly to D&D or have a similar setting; this could include: D20 Modern/Past/Futuristic, Gurps, Gamma World, or Whitewolf.
RPGs that work under a completely different system or setting could include: Paranoia, Carwars, All Flesh Must Be Eaten, Star Wars, and most 'underdog' roleplaying systems.
If you are looking for a computer game to play, a quick google search for RPGs will bring up a mass of results. World of Warcraft, Perfect World, Runescape, any of The Elder Scrolls series, and hundreds of others could all be potential D&D substitutes.
Dungeons and Dragons.
dungeons and dragons
no
Dungeons & Dragons
The Production Budget for Dungeons and Dragons was $35,000,000.
Dungeons & Dragons is not, and never has been, banned in England.
Dungeons and Dragons grossed $33,771,965 worldwide.
As large as the smallest Dungeons & Dragons giant figure.
They entered through a Dungeons and Dragons themed rollercoaster.
Dungeons and Dragons grossed $15,185,241 in the domestic market.
While Dungeons & Dragons shares some characteristics with Alternate Reality Games, it is much better classified as a Tabletop Role-Playing game. There are firm rules in place for players in Dungeons & Dragons whereas ARGs have loose guidelines for them. ARGs also do not require players to assume alternate identities, or even to roleplay, which is the foundation of any Dungeons & Dragons game.
They have Dungeons and Dragons branded games for the DSI. You can buy them at any gaming store. You cannot, however, play tabletop Dungeons and Dragons on a dsi; they are two different gaming concepts (videogame and tabletop game).