The Zoom H2 is a great, inexpensive portable audio recorder. The downside is that its input options are rather limited. If you're willing to take the device apart, however, you can change that.
YouTuber Bertoaus walks through the process in the video above. It's not the simplest hack in the world, but it doesn't look too difficult either. Bertoaus explains the process:
The Zoom H2 has that (hidden) possibility, because it can record the sound of the 4 build-in microphones simultaneous. The only thing we need to do is to construct 4 line input plugs and 2 two pole switches.To change the soundlevel we solder some resistors and condensers in the box. As reward for the modification you get a cheap multi track fieldrecorder in the palm of your hands.
A good four-track portable recorder isn't the easiest thing to come by, even if you're willing to pay more for it, so this is a very helpful hack for those who need several inputs on the go.