Besides being a great research, communication, and professional development tool for teachers, there are four major ways IMHO by which the internet can enhance the teaching of languages. The examples of each, given below, are not meant to be exhaustive, but are meant to suggest a range of possibilities.
Interactive exercises and tutorials for language learning (can use sound, video, images, text). Frees up class time for more communicative activities. Also gives student immediate feedback, frees student from time constraints for completion, and may increase student time on task.
Here are some examples:
C.B. Putnam's Home Page Foreign Language Teaching on the Internet: Resources with teaching activities. Includes specific resources for: French, Spanish, German, and Italian, but the ideas and lesson plans will work for any language.