• https://www.browserstack.com ($30/mo) karma-plugin
• https://testingbot.com/ (free for FLOSS) karma-plugin
• https://saucelabs.com/pricing ($20/mo) karma-plugin
• https://www.lambdatest.com/ (free-tiers 60 mins/mo, then $15/mo)

One issue with SauceLabs is that the karma plugin apparently got outdated (because protocol evolves, etc...)

Now there is also this karma-webdriver-launcher plugins which aims to be more standard via webdriver (which wap them all).
It's said to support SauceLabs and even advised by lambdatest.
Even more interesting (from the above pricing), TestingBot says it's supported by webdrive.io.

@AidasK ?