Cubetutor – The ultimate place to manage your list, allowing everybody to draft it. You can get many useful statistics about your list. It is also a great database allowing you to look at other lists, see the most played cards by set and much more – check it out!
MTGSalvation – The cube forum has an active community and contains everything cube related – articles, evaluation of new sets, a place to publish lists, tips on where to buy cards, and discussions about everything including sleeves and card art.
Cube Design Philosophy – A must-read for new cubers getting into the format. Written by wtwlf, one of the prominent member of the cube community in MTGSalvation. He also writes amazing cube set reviews!
This article by FemmeFatale on tappedout is a very detailed entry-level material on how to even start building a cube. It explains many things other places on the web will take for granted. Long, but a great read for new comers to the format
The Riptide Lab Cube Academy Series – The Riptide Lab is one of the more influetial websites on cubes, it is long running and highly active. I do not agree with most of the common thoughts there, but the canon articles are solid nontheless. A resource to keep in mind, for all things alternative.
This subreddit is the reddit resource on building new cubes.
MTG Cube – An interesting blog that has been active for years, including set reviews and card spotlights
Usman’s Blog – Usman has been writing about cube for years in different places. This blog has links to all of his previous work. He also participates in cube podcasts.
Cubism – This series of articles by Aaron Paquette is rather new and fairly short, but is still good content. He does set reviews consistently.
Mana Short – A very good article by Wtwlf that will explain why you should be very wary of how color intensive cards in your cube are.
Lists of large cubes –
Cubetutor 720 – A cube generated automatically from all cubes in cubetutor, giving the most played cards. Note that it was never built as a playable list, and includes weird choices because cubes of many types are considered here together, but it is a useful reference nontheless
Kenny’s Cube – one of the longest running cubes out there