If you want a general purpose one that can do anything from directional stereo to surround sound, you might consider the
Zoom H2n digital recorder, which is what I use with my D5100. A simple male-to-male 3mm jack cable connects the recorder to the camera, and a
cheap flash mount attaches it to the top of the camera. For a few pounds more, you can get it
with the accessory pack or
buy that later. It's worth having the accessory pack for the mains lead, handle (to isolate the sound), windshield, and remote control.
The best thing about this arrangement, as I've found, is that if you can do audio recordings without needing the camera and without wasting video. For example, it can be great for grabbing some background sounds for atmosphere, or nature sounds for a nature video, and so on.
Having said all this, if you're doing interviews and you can afford it, get a shotgun mic as well; the Rode Videomic is a good one. And if you can afford it, get a lavalier mic as well - you never know when that's going to come in handy. I use
this cheap stereo one but the
mono Rode Smartlav is better. Oh, and if you're shooting/recording out and about, it's best to invest in a suitable
'dead cat'.