I have collected below a list of 13+ Common Entrance revision websites, apps and podcasts. However, this is not an exhaustive list so please contact me if you would like any additional pointers. I can offer more detailed advice on specific subjects, and I have compiled my own revision notes for some of them.
13+ Common Entrance Revision Websites
English: Reading Matters, Grammar and Punctuation Exercises (some quite tricky!) and The Times Spelling Bee. Furthermore, there are some good notes here.
Maths: Mathletics (requires the purchase of an annual subscription) and some excellent revision notes here.
Science: Science4CE and Supascience. Also, revision notes and a large number of multiple choice practice questions here (from Papplewick).
French: Languages Online (French), Target Language and BBC Bitesize (Foundation Tier). Also, practice exercises and revision notes here (from Papplewick).
Latin: Vir Drinks Beer, Cambridge Latin Course and Cyberlatin.
History: History on the Net and excellent notes on this page (from St Hugh's, Faringdon) and here (from Papplewick).
Geography: Excellent notes here (from Papplewick - including a revision guide and much global location practice work) and here (from St Hugh's, Faringdon).
Religious Studies: Common Entrance Religious Studies Revision.
In addition, the BBC website usually has a wealth of appropriately presented and useful information (and games) on a variety of topics; various useful pages can be found here and also here. Of course, these resources are geared around the National Curriculum and not Common Entrance, so you will have to pick and choose a little. Those preparing for a scholarship would be wise to pay a daily visit to the BBC News page; younger pupils may enjoy the Newsround page instead.
13+ Common Entrance Revision Apps and Podcasts
English, Maths, Science, History, Geography, Religious Studies: 13+ Common Entrance revision app by EducationApps. Individual subjects can be added as in-app purchases.
Latin: Pipiatum Level 1, Pipiatum Level 2 and Pipiatum Level 3. Quizlet is also very useful; search for "GilbertWatson" within the app and you will find a large selection of Latin revision quizzes.
Geography: Revision Podcasts from Pembroke House School.
Religious Studies: Revision Podcasts from www.ce-rs.com.
A few other relevant sites include...
The Independent Schools Examinations Board is responsible for setting Common Entrance and Common Academic Scholarship papers.
Galore Park publishes Common Entrance text books. They are also the official purveyors of Common Entrance past papers.
The Good Schools Guide contains a range of help and advice on almost every aspect of education.
Bofa has a range of interactive online resources to help pupils to prepare for 11+ English, Maths, Verbal Reasoning and Non-Verbal Reasoning tests - in so far as it is possible to prepare for these.