Worldbuilding Subreddit: This site was born out of the subreddit. Here you can read guides, join discussions and post your creations. It is a very active community with over 100 people logged in at any one time and hosts a plethora of resources. I highly recommend bookmarking it and using it regularly.
Legends and Worlds: Despite still under development, this is an excellent site for hosting your world and keeping it organised. Everything can be grouped together neatly and stored in custom categories. What really makes it stand out is the timeline tool which allows you to map out events in your world.
List of Price of Medieval Items: Shows relative prices of various goods from the middle ages.
Medieval Demographics Made Easy: A resource for figuring out population densities, population spread, likelihood of various services (such as an inn, tailor, blacksmith etc) existing in a settlement depending on its size. Also covers a few extra things such as agriculture capacity and castles.
A Little Imgur Gallery of Where People Like to Settle: A couple of pointers about positive and negative factors influencing where people like to settle.
Climate Charts: A number of charts showing where various climates/biomes are likely to be with regard to latitude and location (eg coastal, inland, highlands etc).
Native American Dwellings: A map of the USA showing the style of dwelling for Native Americans. Great for illustrating the diversity and different styles.
A Reddit Thread on Plant Colours: An archived discussion on plant colours (contains a number of useful article links).
And Another: Same as previous. Both may be useful for making alien planets.
Prompts and Checklists
The Zaharam-Chapelle-Parunas Ethnographical Questionnaire: A (lot) of questions to get you thinking about different aspects of your cultures. Whether you decide to answer all of them, or just read through when your not sure where to develop. It has categories such as Place, Time, Race & Ethnicity, Family, Customs and loads more.
Culture Making Checklist: A more concise set of prompts to help you develop various aspects of your cultures.
Fantasy World Building Questions: More questions to get you thinking about people and cultures.
Dungeons and Dragon’s Checklist: Another set of checklists to help you develop. Also covers magic and (fantasy) science.
Chaotic Shiny: A random generator for loads of different things such as Civilisations, Holidays, Laws, Rituals, People, Attitudes, Bazaars, Taverns, Currency, Potions and literally loads more.
Lucifer’s Onomasticon: A giant resource for person names (and a bit of history) from cultures all over the world.
Climate Cookbook: A great resource for figuring out climate info.
Fundamentals of Physical Geography: An in depth look at physical geography.
Thanks to Jafiki91 for contributions to this depot