When the Tiny MCE spell check feature stopped working in WordPress, it was looking like we were going to have to install Jetpack to get it working again. Nothing against Jetpack personally, but it's kind of a big plugin and we had concerns about how resource intensive it might be.
Then I stumbled across Mr. Muro's TinyMCE Spellcheck plugin, which was a fork of the original "After The Deadline" plugin . . . and it worked like a charm. Thank you!