most popular software
you probably mean DAWs? sequencing/recording/maybe a bit of midi?
the industry standard is still probably protools.
other high end (expensive) options are cubase/nuendo, logic (esp. macs) and sonar, samplitude, magix....
cool edit pro started off life as
a mastering suite and then became a very popular alround tool because it wasnt one of the big expensive suites... now its been bought by adobe i think. good piece of software
cheaper alternatives abound... such as n-track (which is free i think?) and programs like
intuitive mx...
all these programs have their pros and cons and it really depends on what kind of music you are making...(eg. do you need midi, do you need to do a lot of sequencing, is vst a big deal for you, what platform do you use, do you need surround support... the list goes on and on)
at the end of the day its probably best to start with something cheap or free and wait till you hit its limitations then go from there. (alternatively get a demo version of a more expensive program.... but dont pirate!)
cheers!