This website is an invaluable resource for all things ServiceNow and should be the first stop you make when looking something up, it contains a page for nearly everything that goes on in the system and delves into quite a bit of detail, with decent examples often included.
This should be the second stop you make when looking up anything ServiceNow related, the community is very active and members were rewarded for answering questions- meaning participation is high and the quality of advice is of a good level, you will need an account to create posts and reply to threads.
This is a fantastic site to keep up to date with everything CMS related in ServiceNow, the content is very original and really up to date. The writer works for ServiceNow and creates some cool stuff, this is definitely worth a read if you are developing a CMS in ServiceNow.
SNGuru covers a very wide range of topics and they are all ServiceNow related, the writer is very active on the community and also runs a ServiceNow consultancy. The content is very detailed and precise, I have used techniques listed on this website multiple times when creating solutions or troubleshooting ServiceNow.