*Linguists -> Polyglots.

Duolingo is OK and offers a variety of languages, though take most of their reviews with a thumb-sized grain of salt. Their main strength is nudging you towards regular practice, and that's far more important than talent or motivation. Your resolve might waver, but you have a streak to uphold, so you'll log in and go through a few exercises. It's habit forming. Perhaps the most positive example of something being gamified. They're also great for forums and immersion training in some of the languages (Spanish, Portuguese, German and French IIRC).

My flatmate and his girlfriend use Italki, which they swear by, but I never tested myself.

Really, though, learn anywhere, but practice through immersion as fast and as often as you can. Read stories for kindergarteners and move forward. Watch something that interests you in that language. If you know some nature documentary/Godzilla movie by heart, look if it's available in the language you want to learn. If you're into "let's plays" or cartoons, look for those. You know the drill. Change the language in your OS/browser/whatever program you use most often once you'll feel you know it well enough to switch back if need be. Something I do to keep memorised words fresh is to say them aloud when they come up. Example: when I dress in the morning and pick up my trousers, I'll say "spodnie, pants/trousers, die Hose, los pantalones… etc." Try that for counting, getting change, groceries etc. Those are the words you're likely to need since they come up every day. There's time to know what 'encomiast' means, but it's far lower on the priority list than things you eat. Same goes for grammar. I know most of those weird "needn't have had who hadn't have needed" constructs, but day-to-day I rarely need more than conditionals and simple tenses.