Personally, I like Switcheasy's Thumbtack Mic. It's small, unobtrusive, and I can speak into the iPod Touch as though it were a phone. Its downside, though, is that it uses the headphone jack, so you can't replay and hear what you've spoken until you've removed the mic. Not a big deal for me (I swap it in or out as needed), but I guess it depends on your uses or workflow or simple comfort level.