/r/OnOff is a subreddit devoted to pictures of women with and without their clothes on. The niche didn't really exist for a long time, but after a while people started to realize that the idea of seeing a girl wearing clothes, and then immediately not wearing them was pretty damn hot. Thus, the on-off niche was born and this place is definitely one of the best places to find it.
The subreddit currently has over 150,000 subscribers, making it one of the biggest and most successful adult subreddits around. The mods have very few rules and to be honest, all that's required in order for a post to be good for the stream is that it contains girls with and without clothes on.
There's a great mix of amateur and professional content here, with a clear focus on the former because as we all know, corporate interests and porn companies have done very little to attract attention from Reddit users.