Here are my top websites for worksheets:
Skillsworkshop has worksheets covering ESOL, literacy, numeracy and IT. There is a very useful search option which allows you to find resources by subject area (e.g. speaking, reading and writing) as well as by level.
BusyTeacher has an extensive range of resources for teaching English covering areas such as grammar, vocabulary, speaking and pronunciation. Worksheets can be easily located by subject area, but there is no option to search by level.
EnglishBanana is another site with resources for teaching English. There is a wide range of worksheets organised by subject and level, although the levels do not appear to always match that of common coursebooks.
Inside Out is an English coursebook from Macmillan which has an accompanying website that has an impressive archive of elessons. Resources can be filtered by date, level and subject.