First, before you even think of attaching a switch to your computer, you will need a switch interface. My district will loan out switch interfaces, but it's a NIGHTMARE to check stuff out and turn it back in, so I bought my own interface. I bought this X-keys interface for $53 and it works like magic! Better Living Through Technology has a great post explaining switch interfaces and what you'll need to get started using a switch with your computer. After you have the interface, you'll have to download the software and get it all hooked up (whatever interface you buy should explain this process).
Shinylearning is amazing! The website is made specifically for kiddos with disabilities who use assistive tech (specifically switches and touch screen). The website has simple cause and effect games and also simple single switch games and the website words on computers and tablets. The website has some free games but you can purchase a subscription to get access to more games.
SpecialBites is also awesome! The website has a variety of free switch games/ software that you can download to your computer and also free online games that don't require any download. The website also has a variety of levels of games (some are simply single switch cause and effect, some require timing, and some require multiple switches!). We love the free online games- just go to the website and click "free online games" on the left tabs and then you have a ton of options like sports games, angry pigs, sensory games and so much more! Our favorites are the soccer games, all of the easy fireworks sensory games, and angry pigs.
Although FisherPrice is geared towards infants, toddlers and preschool age students, some of their free online games are perfect for switch toys and are really motivating to my kiddos. From the link, you can click on "online games" and then pick an age range and explore. Two games that work great with switches and that my kids LOVE are the Laugh & Learn Peek-a-Boo Game and the Rainforest Peek-a-Boo Game. They're perfect for single switches/ cause and effect!
If you know other websites that are great for switch toys, PLEASE share them in the comments! I would love more ideas and I know others reading this would too!