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.