There are a number of tools that'll "produce" a DRM-free version of songs. But the catch is they just play the song and record the output, and re-encode it. Basically they provide a quick, automated way of burning a CD and ripping it.
The problem with this, of course, is that you loose some quality when you go to re-encode the song (because MP3 and AAC are
lossy data compression formats). iTunes sells songs at 128Kbps which isn't all that great to begin with, so re-encoding it might make it sound pretty crappy.
There are the 'iTunes Plus' songs that you can get at 256Kbps and DRM-free -- but those are an extra 30c and not all songs are available. If you're in the US, consider using
Amazon's MP3 service which is all DRM-free.
There are tools available to
crack the DRM (decrypt it, to produce a lossless DRM-free version). One such project is called Hymn ("Hear Your Music Anywhere"). Not sure if there are any tools available for the latest version, though. (And apparently it is legal under "fair use" in copyright law).
If you want to go the lossy route, you should be able to find lots of apps via Google. If you're on a Mac, checkout the apps listed on
iusethis. The suggestions of using a virtual CD-RW drive are good, though going through all this all the time might be time-consuming.