Do you know which web 2.0 application to use?

I'm always testing new web 2.0 applications and services and some times I forget why I don't use some and why I use others a lot. So I decided to do the logical, document it. I created a Wiki called Web 2.0 Applications to keep track of these software and services and at the same time add a quick review of each and make it available to everyone.

In my Wiki I explain what that service is, how can it be used in training, and provide videos and tips when applicable. I also list the services I've tested along with a review which includes their ease of use, fees (if any), URLs, if customization is allow, my star rating and any other information I think will be helpful.

Hopefully my trial and error will help others.

Is there any software or services you use or wouldn't use because of a bad experience that you would like to share with us? You can post your reviews here or add them to my Web 2.0 Applications Wiki.