Many players offer FM radio reception, although they're more common on flash devices, which lack the spinning hard drives and the CD-playing features that can interfere with the signal. AM radio is extremely rare. Other radio-related features include FM recording (usually to MP3), transmission (for playing back audio wirelessly through home and car stereos with FM tuners), and presets.

For larger hard drive-based or CD MP3 players that you stow in a bag, an in-line remote control in the middle of the headphone cord is a great feature. Look for a model, such as the iPod, with a clip for attaching the remote to your bag strap. Ideally, it should also have a display with full functionality, such as the one found on most iriver models.

Most MP3 players that record high-quality audio suitable for music recording as opposed to voice memos or interviews do so through an analog line-input, but there are exceptions. A few devices also record digitally or from mike-level sources. The former works great for dubbing tracks from modern stereo components. The latter lets you use a small, unpowered microphone for live recording. (You can record live audio without a mike-level input but only with a powered mike that outputs a line-level signal to the device's line-in jack.) If you want the highest-quality recordings, make sure the player has a decent ADC and can record to uncompressed WAV files. Real-time encoding to MP3 or WMA comes in particularly handy when encoding from vinyl, cassette, or CD without a computer.

Some MP3 players have a small internal microphone for recording the spoken word. These are acceptable for documenting a lecture, a meeting, or a conversation but unsuitable for high-quality music recording. The sampling rate is typically low, and the recordings are mono. The upside is that you can usually record several hours of memos, then upload the files to your PC for attaching to email or archiving.

Some MP3 players can accept phone books and schedules. The information is generally entered into Outlook or another PIM program, then synced to the device where you can view it but not change it on the go. These features tend to be limited and, for most, won't replace a handheld.

Audio purists generally prefer to hear songs exactly the way the recording engineers mixed them and don't have much use for digital signal processing (DSP), which changes a song's equalization or spatial characteristics. But since digital audio files come from multiple sources and people have individual sonic preferences, EQ and other DSP settings can be useful. Look for a customizable EQ setting (the more bands the better; five is the norm), as well as the standard presets (Rock, Jazz, Bass Boost, and so on). In terms of spatial enhancers, DSP features such as the SRS Wow addon can make music coming through headphones sound like it's bouncing around a large room. It's purely a matter of personal preference, but we think music sounds better with some degree of customization.

Just about every player offers shuffle, repeat, resume, and playlist functions. Still, there's room for improvement. Newer models offer the ability to create on-the-fly playlists sans a computer; smart playlists that can serve up, for example, a mix of 1980s hip-hop; song-rating methods in which preferred tunes appear more frequently in Shuffle mode; automatic cross-fading that adds smooth transitions between songs; automatic volume control, which makes all songs equally loud; and other artificial intelligence modes that can make it feel like a pro DJ is spinning tunes for you.

All computers sold today come with a USB 2.0 port. All current Macs and certain PCs also ship with FireWire ports. All modern MP3 players use USB 2.0 for loading tunes from a computer. Some next-generation models even ship with Ethernet, Wi-Fi, and/or Bluetooth capabilities. You should choose a player that has the same type of connection or wireless protocol as your computer(s). If you plan on connecting your MP3 player to your stereo, get a device with a line-out port, or if your stereo supports digital output, be sure your player does so as well. These outputs make songs sound slightly cleaner over speakers.