I don't want to single out one! I have a whole tool kit!
For me personally - Evernote (though having said for me - I used a shared notebook recently to share some information with students), Diigo for my hundreds of bookmarks and its research tools and WordPress - several blogs to share information with teachers and students.
With students - Wikispaces so we can have a space where we all have ownership and can collaborate and Moodle. Moodle for providing information mostly, though the forums have been used a little at school.
As a Mathematics teacher I love all the excellent free resources out there - I'll second Rashmi's earlier recommendation of GeoGebra - there is also a version available with younger students in mind. There are many online resources available for learning about GeoGebra.