Imagine legally filling up your Creative Zen with 32GB worth of music (or about 8,000 songs) from an online music store and paying less than the price of a CD. How is this possible? Well, you're not actually buying the tracks; instead, you're renting them in an all-you-can-eat scenario for a monthly subscription fee. Sounds attractive, no? Welcome to the world of the on-the-go music subscription service. To many music lovers, the "to go" aspect has been the missing link in the music subscription equation since traditional services don't allow you to transfer tracks to portable devices unless you buy them outright. But today, there are two major players filling that void: Napster To Go and Rhapsody To Go. Read on to get our take on the existing services.

A note before we begin: If you're wondering why we aren't including the Zune Marketplace in our roundup, since it does offer a "to go" subscription service, it's because that service is tied to the Zune player. The Zune only works only with the Zune Marketplace and vice versa, so you don't need to choose a service if that's your player. The same goes for the MusicGremlin's store and "to go" service. Then of course there's Slacker, which is a horse of a different color.