The www.dreamcricket.com game is free and easy to play, they run frequent competitions covering a particular tournament. They have been running competitions for years and sometimes give out prizes.
The www.cricketinc.com is more complex but offers the biggest prizes of any free fantasy cricket game. It can be quite rewarding to learn.
The www.cricketweb.net site has a new format for free fantasy cricket games and has a new competition starting on the 6th of Feb 2006. An improving site.
The www.sportal.com.au site has rare virtual cricket competitions, usually covering Australian home series.
The cricinfo site occassionally has a fantasy game but it has a poor interface and is not very interactive.
The www.raindelay.net site occassionally has games using tests involving Australia.
The www.testskills.com site has some fantasy games.
The www.fantasyfactory.co.uk usually has a free fantasy game covering England test and ODI series.
The www.bigcricket.com forum has occassional fantasy games and a thread on free fantasy games.