Try MP3 Music Downloads in the Market, havent used it, but looks ok. You may want Album Art Grabber too, in the Market. Both are free.
If you have music on your computer just click and drag from Windows Media Player to your EVO connected via USB. Or just click and drag from the folder you keep your music in to your EVO's music folder. Select "disk drive" when you connect your EVO if you're going the second route.