Games like Rockband, Guitar Hero or DJ Hero are popular these days, it gives people who otherwise have no interest in learning an instrument the feeling to experience how it is to "play" music in the most basic meaning. You have to push the right buttons in the right pattern and thats it. Now the above mentioned games are commercial ones, but there are free alternatives available who often outsmart the originals because they have a force behind them which companies don't: A community who is also interested to create content as well as modifications. I am going to show you a few examples.
Stepmania The original "Dance Dance Revolution" game appeared on the surface of the earth in the year 1998. The mechanics are simple: 4 arrows (up left down right) show on the screen and the player has to step on the right fields on the controller which is a dancemat. That means you have to step on the right field and dance your way through a stage. In the year 2005 a free open source clone of this game was released: Stepmania.
The game is practically the same as DDR, again it is about stepping on the right arrows but something was added: Stepmania now also supported controllers and keyboards. What this means, as well as stepping on a special dancemat players not could play the game with the arrows on their keyboard which became an often more used method of controlling this game. Many players developed an almost unnatural skill with this game. There are many extensions available as themes, songpacks, ingame announcers etc and even new playmodes being implemented. Download it at Stepmania.com
Frets on Fire When Guitar Hero hit the shelves in 2005 this was the most easy way to feel like a Rockstar. The player got a mini guitar (not a real guitar, though, it has the shape of a guitar but works with buttons that have to be pressed) and easily could play through rock music classics. Following the example of Stepmania not long after a free clone for the PC was released: Frets on Fire.
The game supports as well guitar controllers as also keyboardplay, where the F keys substitute the buttons on the controller. Download it at Frets on Fire's Sourceforge page.
Doremimania/Synthesia In 2000 a game was released that came closer to playing a real instrument than many games until this very day, it was called Keyboardmania and featured a little real midikeyboard as controller. Practically the player would play through a song as on a real pianokeyboard. Yamaha released a PC version which was shipped along with the EZ-250i keyboard. The instrument had glowing keys which would also lighten up when you need to play them and it was advertised as a fun method to practice. As apparently the game looks especially for this Keyboard and does not even start up if it is not connected there is no way for non-Yamaha owners to play it. There was a clone called Doremimania which apparently is abandoned now, it is hard to gather official news since all these sites are in asian language
(japanese/chinese? It is even hard to get that out with no clue of the language). There is still a download site available at http://www.geocities.jp/torimaxholdings/frojectd.html which might or might not be the official site. You will also need some PMS files to play (on there too).
Later on another game appeared on the internet which tries a more western approach: Synthesia. It does not depend so much on the players ability to have a clue how to play initially but puts more emphasis also on the learning and practicing. A free "game only" version is available which has an unlockable learning mode that shows also real sheetmusic, adds a waiting function so the game does not continue until you pressed the indicated keys and more.
Get it on the official site Synthesia's official internetpage. Conclusion: Those games are fun to play but hardly anything comes close to play a real instrument, only the pianogames are very good if that is your goal. But then those are for free and its damn fun to play! So go on and try it if you like