Some of Chegg's competitors are Studyblue, Course Hero, Slader, and Cramster. However, Chegg is the market leader by a mile.
It's a little difficult to sort out which of these businesses actually sell solutions, because they don't admit they do that. For example, the NY Times did a softball interview recently with Chegg's CEO, where they asked, "Many teachers believe that their students are using Chegg as a means by which to cheat. Is this a problem? And if so, what are you doing about it?" He answered with lies and evasions:
It’s always been a problem for colleges. Let’s face it: Students have always found a way, whether it’s in fraternities, or whether they go to Google. But Chegg is not built for that. We have built technology that removes copyrighted material before it even gets posted. If we’re notified by a professor or a school that there’s copyrighted material, it immediately gets flagged and then removed.
(None of this is true. Chegg is built for that, and their entire business model is based on illegal use of copyrighted material, i.e., the questions themselves. The publishers tried to get Chegg to take down copyright-violating material, and ultimately gave up because it was like whac-a-mole.)
I would actually love to see a careful, impartial analysis of which web sites do what and how their business models work, but I haven't been able to find one. It's pretty difficult to know exactly what they're doing unless you pay for an account yourself. But just anecdotally, in my department the issues are 100% Chegg. It's especially bad now that we have the campus quarantined, so that all tests have to be take-home. One of my colleagues found all of his finals on Chegg last semester. He ended up not grading the finals and assigning projects instead. The consensus in my department seems to be that starting next semester we'll start just doing a half-hour, synchronous quiz every week, instead of big exams. This will hopefully make it harder for students to post exam questions on Chegg and get answers in time.
In comments, Peter Saveliev wrote:
They are just exposing some pre-existing problems in academia.
I don't think this is true at all. When I started teaching in 1996, problems with students cheating on homework were minor and intermittent. It was easy to nip it in the bud if you cared enough to read any student work. Today it's a qualitatively different environment. In classes where professors continue to count homework as a significant portion of students' grades, cheating is nearly universal. It's very corrosive, because students come to believe that they can't compete on an even playing field if they don't cheat.