I use Anki. I wouldn't use it in the beginning, but when you're at the point that you can listen to some simple podcasts or videos or read some simple texts, then when you hear/read a sentence with a word or two you're unsure of, you add it to Anki and you get spaced repetition on that. (Don't just add lone words – although the context may feel like cheating, you want that reinforcement of related contexts.) Often I'll look up a word and find some real-world usage example too and add that.
I did use Duolingo a bit when starting out, but I felt like I didn't really remember any of the words introduced there, just learnt how to hit the buttons as fast as I could. (And oh gods those annoying animations). Pimsleur on the other hand is amazing when starting out. Then a combination of podcasts, simple stories and Anki.
I've also heard good things about italki, seems like a marketplace for 1-on-1 language tutoring online.
Piggybacking off your Anki mention, I'd like to continue to spread around something (currently only of interest to people learning Italian or Spanish) that I've been up on lately. A guy came up with a wonderful (and monolingual) deck format for people who are learning Spanish or Italian to use the old-school method of learning Latin where you learn all verb conjugations by rote first and get them (and tenses/moods in general) out of the way. Then you can dedicate your language learning time to acquiring words, sentences and colocations (i.e. interesting stuff) rather than repeatedly and incompletely relearning the verb mess, leading to permanent blind spots.
He also surrounds the verbs in mock clozes, so you get hammered with context. You can also do it without doing anything else; the Romance verb situation can be isolated from the rest of the language completely. At least for Spanish you'll eventually need to know what common subjunctive triggers are, that they'd rather use the present tense (with context) than the future or past more often than you'd think, and other stuff that's an order of magnitude simpler to remember than those damned verbs. If you need to know irregular vosotros conjugations in subjunctive future, you've already got them in your head.
I did use Duolingo a bit when starting out, but I felt like I didn't really remember any of the words introduced there, just learnt how to hit the buttons as fast as I could. (And oh gods those annoying animations). Pimsleur on the other hand is amazing when starting out. Then a combination of podcasts, simple stories and Anki.
I've also heard good things about italki, seems like a marketplace for 1-on-1 language tutoring online.