One of the things I am doing this summer is brushing up on languages. Something I came across that has turned out to be quite useful is Duolingo, which is free and actually quite effective. Each Duolingo course aims to cover roughly the same ground as about a year at the college-level, and does so mostly by drilling dressed up as a game. It's especially good for a refresher. I started the French course on June 1, and am about two-thirds of the way done, and have found it reasonably good at stitching up patches in my memory. I liked it enough that I've also been taking the Bokmal (Norwegian) course, to see how useful it would be for learning a language almost from scratch (I knew a few words and phrases of Norwegian, but that's about it), and while this is obviously a much slower process than simply using it to review, it seems to be reasonably effective. I started the Bokmal at the same time as the French and I am about ten percent of the way through; but already can handle basic and generic sentences, at least, like Mange lærere leser bøker, "Many teachers read books". I'll be starting the Spanish tree a little later this month, also for a review, and the German tree (my German proficiency would be somewhere between my Norwegian and my French proficiency, since it's more advanced than nothing, but very patchy) a little later.
The languages they currently have are: Spanish, French, German, Italian, Portuguese, Dutch, Irish, Danish, Swedish, Esperanto, Turkish, Norwegian, Ukrainian. There are several others that are in the process of being developed. There will eventually be a Vietnamese course available, so probably this fall I'll start using it for my on-again-off-again Vietnamese study. The courses are all crowdsourced, which means you occasionally run into odd translations, but this hasn't been a serious issue.
To brush up on Latin, I've been using some of Evan der Millner's YouTube videos, particularly his Latin in Latin course (Part I, Part II, Part III, Part IV). He summarizes his approach here.
There are other online resources, of course, like this Norwegian mini-course that I'll be starting next week, as well as standard offline book approaches (e.g., I'm currently reading through Lingua Latina Per Se Illustrata), but I've found these to be both reasonably accessible and useful. So what online language resources have you found useful?