Edmodo: an education-based social networking site, with a layout similar to Facebook. The teacher creates a group, which students subscribe to with a special code. Then the teacher can upload assignments to a calendar, post a poll, question, or discussion topic, and the students can see and respond to the posts in real-time. Best of all, the service is free!
Edublogs: the most widely used blogging service designed specifically with teachers and students in mind. Free to use.
Facebook: while some may question its value in the classroom, Facebook can be a great (and free!) tool for connecting with students.
Ning: create your own social media site about any topic. Basic service is $19.95/yr.
PBworks: a collaborative website building tool. Teachers can set access rights for students in their classrooms to upload to or modify the class website. There is a free basic service, but the classroom edition is $99/yr.
Weebly: a free website or blog maker. You can choose from hundreds of professional looking themes, and the sites are easy to build and manage. One of the only website makers without forced advertising!
Wikispaces: create wiki pages for any topic to use in your classroom. Standard service is free, other packages vary in price.