There are probably more web-based language sites than we could possibly link to. Most of the ones I know are Spanish, since that’s the language I’m trying to learn. Some are for multiple languages. Please recommend your favorites, and we’ll try to add them.
Live Mocha is a community-based site that offers multiple languages. It has many learning and practice features, but also offers help from other community members. So I help folks who are trying to learn English, and some Spanish-speaking people help me with my Spanish.
Spanish Language
Intro to Spanish features tutorials, games, vocabulary, and other help.
Spanish Spanish has lots of games, flash cards, vocabulary and grammar help. Some free, some by paid subscription.