From Kate Ahearn's blog

Update: Owlie Boo one button activities and ePlayground have been added to the list

Cause and Effect (and Press to Play):

• Bugs, Transportation and Musical Monkey  (set switch to tab)
• Help Kidz Learn*
• Hiyah*
• Kneebouncers*
• Priory Woods Videos Revamped*
• Learning Tools (scroll down)*
• Owlie Boo* (any of the activities under "Pressing Keys" with switch set to space bar or enter)
• Rock Paper Scissors
• NGFL Snap Matching Activity (teacher must set up, but arrow over the "Snap" button)
• Silly Books (look for the "click through books" and set mouse arrow over "next")
• Special Bites*

Single Switch Scanning/Timed Single Switch Response (sites may include some cause and effect games as well):

•  A Sliding Puzzle for Helen (download)*
• Bullseye
• CBBC*
• CBeebies*
• Cow Jumped Over the Moon 
• ePlayground (four completely accessible games)
• Help Kidz Learn*
• Jack Be Nimble
• My Switch Games by Scott McKay*
• NGfL Switch Coloring*
• Papunet*
• Priory Woods Kids Only*
• SEN/Switcher*
• Shiny Learning *
• Special Bites*
• SwitchedOn Games Bowling 
• Switch Music 
• Tar Heel Typer (online switch keyboard) 
• The Great Fish Race (download only)

Two Switches (unless otherwise noted you must click in the area of the game and then use tab and enter to play):

• Accessible Book Launcher*
• Bug Memory
• Fruit Memory
• Space Memory
• Sensory House (tab and enter)*
• Special Bites* 
• Tar Heel Typer (online switch keyboard)

Subscription Sites with Free Trials

• Busy Things is a switch accessible site for young children out of the UK.  You can try it out for free for two weeks.
• Switch Climber* (costs $14.95 a month and you can "try-before-you-buy" with a free two week subscription.) 

Also be sure to check out Game Base and One Switch for other (usually more advanced) switch games.

*Those sites with a star have multiple activities to try out!  Wednesday's With Sam lists these by activity instead of website if that is useful for you.