Search google for "free game assets"
But if you want to try and develop your own art, the best FREE software in my opinion is GIMP.
Of course if you're willing to buy softwares I would tell you to go with adobe photoshop, illustrator or fireworks.
Another cool tool to develop your game animation and even your own games is adobe Flash.
And for your music and sound effects, I don't know any free softwares.
I use FL Studio, it's a quite simple software, of course I'm not a great musician so a professional software like Ableton would be too much for me to handle, so FL studio reach all my needs.
But in the end, there's no such thing as a good or a bad software, a good art comes from the artist, so the best software is that one that you know how to use properly.