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 for organising all my bookmarks and highlighting and annotating web pages. The list facility is excellent. Joining Diigo Groups has led me to resources for my own as well as my students’ learning.
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 because I can very quickly and easily create a screencast and communicate some mathematics (mathematical notation can be slow to type even if one is adept with LaTeX).
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 – for creating a social network easily and for me most importantly – connecting with like-minded groups of individuals – great for my own personal development as well as giving me useful ideas for my teaching. Update – this is no longer free.
Google Docs for collaborating on line. 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.
LiveBinders seems excellent – I can easily direct students to a few selected websites, for example see this binder including some favourite websites on equations and a screenr recording of how to use one of the sites.