My personal favourite has to be Evernote which is ideal for storing notes with links and details of favourite sites.
Pinterest – very easy to use and very popular. You will find collections from educators (along with kittens and shoes and ….)
Pearl Trees – visually very attractive and possible to add a good structure.
Another application to achieve this is Sqworl. This site requires registration.
Each link is presented as a thumbnail and it is possible to give brief notes for each as well.
For example: see this collection of formulae for Shape, Space and Measure required for the GCSE Mathematics syllabus my students study.