It's common to record separate sound on digital devices these days. Unlike analog audio, digital audio is self-clocking: digital audio by its nature contains synchronizing information, so that playback happens at the same precise speed as recording. Maintaining a common start mark between picture and sound remains an issue; unless the digital format includes timecode, you need a clapper or other start reference to match picture to sound.
Digital Audio Tape records 44.1 or 48 kHz, 16-bit uncompressed stereo audio on a 4mm tape cassette. Interface is via analog RCAs or XLRs, or by digital S/PDIF connections. DAT gives very clean audio assuming the microphones and preamps are of high quality.
Pro DAT machines may include a timecode track so that you can marry field recordings to picture in a double-system recording setup without the need for a clapper or other common start mark.
Sony's MiniDisc format records 44.1 kHz 16-bit stereo using Sony's proprietary ATRAC codec. Each new track is individually addressable, recordable, and erasable, and the recorders are small, lightweight, and fairly sturdy, so MiniDisc has found favor as a location audio recorder, often in situations where you would otherwise use a wireless microphone. A MiniDisc recorder is simply stuck in an actor's pocket or gaffer-taped to his body, and a wired mic is attached to it.
Portable consumer MiniDisc recorder outputs tend to be analog stereo miniplugs only, as you'd use with headphones. Professional MiniDisc recorders and home units (not portables) add S/PDIF, RCAs, and/or XLRs for input and output, allowing more connection flexibility. Some even include USB for direct connection to computers.
Flash Memory and Hard Disk Recorders
Small recorders using flash memory or hard drives are also filling the need for cheap, portable, rugged location recorders. They run the gamut from cheap consumer devices recording MP3 audio only, with nothing more than stereo miniplug outputs, up to high-end consumer and professional devices recording MP3, WAV, or BWF formats and offering a full complement of analog and digital I/O connections.
Most of these machines include a USB connection for quick transfers to and from a computer. Some even have FireWire. You can mount these devices on the Mac desktop as drives, and drag files to and from them.
Multi-Track Disk Recorders
Film audio is increasingly turning to multi-track hard disk recorders, allowing you to record multiple microphones or submixes on their own tracks simultaneously. Like their lower-end cousins, they offer the choice of compressed or uncompressed formats, and most include USB or FireWire (or both) connectivity to move the recorded audio to a computer for further production.