Here are some of the sites I'm visiting the most this month:
Kwiziq Spanish and Kwiziq French
I don't do a lot of the quizzes (although I could be doing more than I am, I should do that), but I find the grammar explanations and the example sentences really helpful. Some sites have example sentences that seem like things that no one would ever say, or that don't really illustrate the grammar point very well; Kwiziq does a really good job here.
Gosh, do I ever love Linguno. I love that I can use it for so many languages (lots of sites have no German or Italian, but Linguno does ... I do wish it had Japanese, though). I like the ability to create all kinds of sets of verbs to practice, including the starter packs of the top 25, 50, or 100 verbs in each language. This is a great site for getting in five minutes of quick practice during a break between projects.
I'm new to Speechling - I just signed up in the last week or so. I like the variety of languages (I can practice Spanish, French, German, Italian, and Japanese!), and the practice interface is decent. It would be nice if it remembered levels in different languages (if I start with Advanced French and then switch to Japanese, where I'm a beginner, it doesn't remember that; I have to switch every time), but that's not a huge problem. I'm glad it's easy to use it for practice even without doing the spoken practice. I find it helpful to get feedback on my pronunciation from time to time, but much of the time I'm just checking how well I understand certain concepts or vocabulary, so I can just speak an answer to myself and compare it to the site's spoken response. Speechling works well for quick breaks, too.
Genki
My very favorite language practice site these days is Genki Study Resources by Seth Clydesdale. Not only is this a fantastic set of exercises, done beautifully, with a great interface - it's also open source software, with the complete source available at Github. It's also easy to run it offline. I love this project, and I love practicing Japanese at this site. I am so grateful to Seth for creating this fantastic tool.