Expanding my vote to TabIt, Finale, Sibelius, Reason, Anvil, AND Garageband, because Garageband is pretty sick.
Oh and sure, Frootyloops is great for making music with neat sound effects and such, but if you're a real composer, and compose for real instruments (and not all synth instruments) then a program that uses standard notation is definitely the more professional and accepted format. Hell, these days, finale's .mus file is actually an accepted medium of music, along side mp3s and midis. And the reason for all of this is, to actually be a good composer, you have to take into account what instruments can and cannot do, and you have to make it so musicians can actually read and perform your piece the way you want it performed. And most musicians read standard notation.
Because come on. I'm not gonna be playing any of the symphonies I write myself. Conducting, maybe, but...
Yeah, I use TabIt and Finale most of the time, now.