Wikispaces - A fairly good wiki site.
PBWorks - Better than Wikispaces. Very user friendly. It's what you are looking at right now! Check out the PBWorks Educator Community Site for ideas on how to use a wiki in your classroom.
50 Ways to Use Wikis for a More Collaborative and Interactive Classroom
Wikis We All Know
Classroom Wiki Examples: Mr. Boyers 5th Grade Class, Miller's English 10 Resource Wiki, and Mrs. Shanahan's Wiki
Blogger - The most widely used blogging service. Tons of features.
Wordpress - Another very widely used blogging service.
Edublogs - Lets you easily create & manage teacher blogs, quickly customize designs, and include videos, photos & podcasts. Great for communicating with parents and students.
Kidblog - Have your students create their own blogs. Use them in place of journals or other writing activities. Kids love to write online when they know they will have an audience. You can also embed any glogs, vokis, prezis, or videos your students create
Blog Examples: Miller's English 10 Classroom Blog, Gladly Would I Teach, Mr. Cappello's Online Classroom, Mrs. Gay's Science Wiki, and Your English Class