This is very much a personal list – from a teacher who enjoys learning as well as hopefully helping students learn!
WordPress itself! For ease of use and reliability. One can set up and maintain a blog easily and for free. The free version has no adverts, just the modest line at the bottom of the screen. I believe this is an excellent model – the free version being so full- featured and more than adequate for many users with even more features available for those who do require them.
Diigo – a site I formerly used for bookmarks
Twitter for all the useful contacts I have made, being very selective in who one follows can help create an excellent network, good for professional development.
Screenr – a former site for creating videos
Moodle – used for our school VLE, I have a courses for each year group both acadmic and support, easy to use and open source.
Quia – my top ten list would be incomplete without the site that I was using before the school had even heard of Moodle, I can produce a web page so easily with this – message, links, activities and one of the most reliable sites I have ever used.
Ning – formerly used for creating a social network easily
Google Docs for collaborating online. Great for collecting data for a statistics lesson for example – create a form which all the students complete – all the data will be collected on a single spreadsheet.
Wordle create attractive posters of word clouds, with many applications for education.
LiveBinders seems excellent – I can easily direct students to a few selected websites.