Both interactive and non-interactive exercises are available on the Internet. Interactive exercises allow learners to submit their answers to be corrected within a few seconds by a remote computer. There are two types of non-interactive exercises: the first type requires learners to think of the answer and then click on the button next to the question to find out the answer, while the second type requires learners or their teachers to print out the worksheet for them to write their answers on (if answers are provided they are on a separate electronic page; sometimes no answers are provided). 

Some of these exercises are available from: 

1.  CNN Newsroom and Worldview for ESL - vocabulary and grammar exercises based on current issues (Links for students) 

2    Self-Study Quizzes for ESL Students - over 100 exercises on grammar, reading comprehension, vocabulary/idioms, writing (http://www.aitech.ac.jp/~iteslj/quizzes/index.html) 

3.Fluency through Fables - reading comprehension, vocabulary,written discussion exercises based on fables (http://comenius.com/fable/complete.html)

Ideas for conducting lessons can be found from the many lesson plans available.For example, Links to ESL/EFL Lessons & Lesson Plans provides access to lesson plans such as “Newspaper Conferencing” and “Student-Centered Conversations”. In addition, there are language games on the Internet.
Some of these include: 

1Grammar Safari - hunt for the target part of speech 

2.ESL : Quizzes There are currently 5879 registered links. - with many categories.

3.EnglishZone Grammar

There is even a web-site called Brendan’s On-Line Anagram Generator which teachers could use to custom-make their own anagrams.