I like
Japanese Online Institute.
I usually buy 30 or so flex tickets, and do the group lessons. Sometimes, there are 4-5 people, sometimes it's just me and the teacher. (I haven't been on there for a while, but I'm going back this week to ramp up for N1.) Private lessons cost more.
The more you buy, the cheaper it is per ticket. (But check expiration dates.) I think it runs to about 750 yen or so a session for me. They have a 3 lessons/900 yen intro offer to see if you like it.
It's set up so you can just drop in whenever, learn stuff, and move on. If you miss a day, a week, or a month, it's no biggie. Lessons repeat after a while, so the one you missed will come back.
Here are the classes they offer by category: (They're conversation classes unless it says otherwise.)
Introductory, Beginner 1, Beginner 2, Beg 2 Grammar (incl. JLPT N4), Low Intermediate/ Low Interm. Gram. (incl. JLPT N3), Intermediate, Intermed Grammar (incl. JLPT N2), Advanced Grammar (incl. JLPT N1), Advanced, JLPT N1 Kanji, JLPT N2 Kanji, JLPT N3 Kanji & Vocabulary, JLPT N4 Kanji & Vocabulary, and they have a JLPT N1-N5 review/prep course.
It's all done in a group voice-chat Java app. So nothing to install, either, AFAICR.