What are some good sites you use that have a low time impact?

Weird way of phrasing the title I know. I work in customer service at a small company so I sit at a computer and wait for incoming calls. I would say on average I am on calls for 35 minutes out of each hour. The rest of the time I am just sitting there waiting. I was wondering if there are some good WO sites that I can get on to maximize my downtime? I obviously can't do things like Usertesting during that time because I have to be on call. So far I have found Prolific to be a good option but am looking for more. Thanks!