These games are so addictive. So far my favorites are:
- slither.io
- agar.io
- xxxxxxxxx (removed because it now sucks)
- diep.io (tank battle game that involves level advancement and upgrade choices, not in perpetuity, but separately each time you play)
They don't satisfy the same video game itch that battle forces does, but they are great for killing time.
P.S. If you play any of them I hit up slither.io the most but my name in all of them is always Hot Soup : ) without a space between the colon and the parentheses (If I type it like that here I end up with an emoticon like this 
).
EDIT: #3 on the list, lazerdrive.io, changed. Not fun anymore. Don't waste your time.