Gift certificates are often a popular choice for gifts from adults to kids of all ages—even from kids to kids—when you're at a bit of a loss about what to buy. But when a reader asked how she could go about buying a gift card for her grandson so he could buy music for his MP3 player, it occurred to me that not everyone knows how and where to buy gift cards for music downloads. And, more importantly, which music downloads play on which music players.

So here are a few gift card/certificate choices for all kinds of music players:

iTunes cards - Probably the most popular gift cards are the multi-colored cards for iTunes downloads from the iTunes store. The cards come in $15, $25, or $50 denominations, while e-certificates, which can be emailed directly to a gift recipient, can be between $10 and $500 in whole dollar amounts. The gift certificates can be used on anything in the iTunes store, including music downloads, TV shows, movies, podcasts, games, and audiobooks. But this is important: iTunes downloads play only on Apple players, including the iPod shuffle, nano, and video players.

Wal-Mart music downloads - Wal-Mart has a music download web site for WMA songs that play on most MP3 players that are not iPods or Microsoft Zunes. Songs are 88 cents each, and you can purchase a Wal-Mart gift card to use to purchase songs from the music download store.

Yahoo! Music gift certificates - You can purchase gift certificates for a Yahoo! Music subscription or for downloaded music. Yahoo! Music also has WMA (Windows Media Audio) files that require the Windows Media Player and a PC to run. Music download gift certificates are available in $10, $20, $30, $50, $70, and $100 denominations.

Zune cards - Microsoft's Zune player plays songs only downloaded from the Zune Marketplace, which is accessible through the Zune web site. Gift cards are available at stores such as Circuit City for $15, $25, or $50.

There are prepaid cards for other music download services, including Napster, Urge, and eMusic, as well. So when you ask which music player your gift recipient uses, you may as well ask what paid music services he or she prefers, too.

Lots of kids get their music through peer-to-peer file sharing programs, but my guess is they will never turn their noses up at the gift of legal music downloads.

Related: 

iTunes: How Do We Compare Thee? 

Amazon to Sell DRM-Free MP3s? 

P2P Primer for Parents