That brought some new attention on this peace of hardware thet was used in some of the most famous tunes of all time, and is still used today by a lot of producers. Originaly designed by Tadao Kikumoto, and introduced by the Japanese Roland Corporation in 1984, this machine was revolutionary, as it was the first MIDI-equipped drum machine. Partly analog, and partly sample based it had a unique sound, witch is copied still today. Originaly the machine was very cheap in comparation with some hi-end alternatives, but the fame of 909 came with the rise of acid house and techno. 909 allowed the artist in this genres to do a lot with a small investment. The list of the artist that used it, and still are, is very impressive: 808 State, Inner City, Frankie Knuckles, ATB, Aphex Twin, Chemical Brothers, Dennis Ferrer (has even requested to be buried with his 909), Daft Punk, Fatboy Slim, Kerri Chandler, Jeff Mills, Moby, Modeselektor, Orbital, Plastikman, The Prodigy, LCD Soundsystem, Vince Watson, Radiohead, Underworld... The machine was also used by some of the mainstream artist as well, such as: Kanye West, Phil Collins, RZA...
Jeff Mills on Roland TR-909
Basically, the sound possibilities were limited, but you could do so much inside this limitations. Drum sounds are generated, while the hi-hats and cymbals are sample based. The machine is easy to program cause of the obvious 16 note parttern in front of you and dedicated knobs for controling the parameters of every instrument. The analog drums offer a bit more tuning (pitch, attack, decay), while the sample based ones are more limitted. There is also the "accent" option, that is used to emphasize some notes, and give a more "human" touch to the beat.
Roland TR-909 is definitely one of those machines that stood the test of time, and exceeded the results that were expected many times over. If you are the one of the lucky ones that have this machine, then you are well aware of this, and if you are not, you can play with some of the software emulations of it, the quickest way is with the browser based one.