Rosetta Stone: Simply one of the best! Interactive, easy to use, and best of all they have a large variety of languages featured.
DuoLingo: For the person who likes a game interface. Not quite as complete and intensive as Rosetta Stone, but also a large variety of languages (including Klingon and Latin!)
Pimsleur: A tried and true classic!
BeelinguApp: If you love reading books, listening to podcasts and audio, this type of language learning may be for you!
Grammar Girl Mignon makes Grammar easy, fun, and quickly accessible!
Thesaurus: An easy rabbit hole to fall down as you go from link to link with word meanings and relationships!
The Punctuation Guide: Oh, what fun! Do you know when to use a colon? A semi colon?
The Grammerly Blog: A nother superb grammar resource, I've used their extension for years and wouldn't write without it!