EnglishTown
Englishtown is the world’s largest website in the field of "learning English online". The site offers extensive free resources for learning English, including onsite activities and teacher-led classes.
GlobalEnglish
According to their tagline, they are "improving Business English communication skills of global companies".
Spotlight
This is the online version of the SPOTLIGHT magazine. With a good language section where you can find tests and exercises.
Dave Sperling's ESL Café
The internet's meeting place for ESL and EFL students and teachers from around the world.
Do you know or use another useful online service that should be featured here? If so, leave a comment and let us know about it!