I've been on a handful of times, the craziest thing I've seen is there was a link on a website that just read "Links" (to reach any site on the deep web you have to click on a link / copy and paste a url you can't search anything as far as I know) it lead to hundreds of different names some being owners of large companies, people in the government, and others that I have never heard of. Once you click on a name pages of documents open and it's basically incriminating or secret information about that person, with a few paragraph blurb that sums up the documents written by the author of the site. For example one of the people posted was a member of the FBI (I don't remember the name) but in the documents it should that he had like over ten hate crimes filed against him, and what was super weird was all the people he did the acts against were asian.
Also there's basically every fetish porn imaginable, mutilation, really weird animal shit, furry stuff, kid napping forums, the weirdest shit you can think of, a lot of the gore and more fucked up pictures you've seen were initially found on the deep web and then posted on 4chan or whatever and then it spreads from there. The drug sites exist and they are widely used, "The Silk Road" is the most popular, with these kind of sites you usually have to be invited in or have special clearance and once you are in it can take years of being a member (seriously years) before you gain the trust of the other users and they allow you to start selling/ buying products. These kind of selling and purchasing sites exist for anything imaginable. A pretty funny not as serious one is this site where you pick some bullshit thing you want, like an iphone or something and someone steals it and sells it to you for a fraction of the price, when I was on that site they were trying to sell thousand dollar remote control helicopters that they stole for like 50 bucks.
There's all sorts of other shit on there that I haven't really seen proof of or anything but hear it talked about, like alien related shit, mass company mergers, nuclear weapon sales, ect ect.