- WFWSE frequency lists
- Children's Printed Word Database
- British National Corpus (for sample sentences)
I found two particularly useful. The first, and perhaps the most authoritative, is based on the British National Corpus. You can either go there directly by clicking on frequency lists, or get to it through the main site at http://www.comp.lancs.ac.uk/ucrel/bncfreq/.
The British National Corpus includes a wide variety both written (90%) and unwritten (10%) material. The nice thing is that you do not need to download any files to use it. The disadvantage is that it focuses solely on British English.
Note that many of the frequency lists have two versions, by lemma and not lemmatized.
A lemma is a headword in the dictionary. This is the "main" entry that you will find. Other forms of the word will be grouped underneath it. On the lists, a lemmatized word means that all forms of a word (derived or plural) are counted as a lemma or headword. For example, when you check the Frequency list of verbs (by lemma): list, you will find no entries for woke, but a frequency count of 41 for wake. All other forms of wake were counted as wake. The not lemmatized versions of lists simply count words as they occur.
The second list is the Children's Printed Word Database based on reading material of children between 5 and 9. Developed by the Department of Psychology, University of Essex in the UK.
Finally, if you would like to find sample sentences using a word, you can use the British National Corpus website. The source of each sentence is given (book, magazine, newspaper, etc.).