I have the D5100, and at the recommendation of a couple of others on TPF, I bought the Yongnuo 560 and a couple of cheap wireless triggers also Yongnuo, but don't remember the model # right off hand, and I'm too lazy to get up and go look).
I've been completely happy with the YN560. I had (still have, really) NO idea what I'm doing with flash, so it IS a learning process, but the flash and triggers are easy enough to use off-camera. I bought a hotshoe adapter for a cheap old lightstand I had; I put the hotshoe adapter and the receiver trigger on the lightstand pole, then put the flash on the receiver, and another receiver on the camera's hotshoe, and I'm good to go.
Figuring out where to place the flash, and what amount of flash to use, that's another story...and since I can't get the TTL info, I *have* to set it manually, but as far as I'm concerned that's actually a PLUS because it forces me to learn what I'm doing and not just rely on the camera to do it for me. I shoot in manual, because I'm not going to trust the camera to choose my settings for me, so why not use manual flash as well?
If you're on a <$100 budget, I highly recommend the Yongnuo 560 flash!