The official sims 3 Site. Yes. it's still here! As of the date of this entry, June 2017, the store and the exchange are still working. I had no idea how much additional content was available. At the store, there are whole worlds, premium content like the Baker's Station and the Old Mill Waterwheel, clothes and furniture sets.
Yes, it still costs money, but check out the forum for the most economical way to buy store content.
The official sims 4 Site. At the moment this looks mostly like an advertising page for those who do nont already own the sims. However, there are some links in the top menu to the gallery and community.
Introversion's Prison Architect's Official Site (not EA!) Note: you can also purchase just the Steam version through Steam.
So erhm, apparently BlueFang (the developer of the Zoo Tycoon Games) is no longer. Also, Microsoft Game Store has no official site for ZT2 anymore. It is
still buyable from on-line retailers if you do a search. In the meantime, I linked the wikipedia page.
The new microsoft site for everything Minecraft.
Sites with both Sims 3 and Sims 4 DLC
Huge free custom content forum/site for Sims2, Sims3 and Sims4 with original content, plus a wiki, tutorials and discussion.
The Sims Resource is one of the first sites to offer downloads for the Sims and they are still going strong with tons of content for your games. There is a subscription option called VIP to get rid of the ads and give you a few extra niceties.
Around the Sims has been "around" for I think all the sims versions. Click on the links at the right top of the page for Sims 1-3 content.
Simcredible has new Sims 4 content that is really pretty amazing. Although they do not seem to be adding new Sims 3 content, they still have an extensive volume of CC for Sims 3 as well.
Some sets are by donation only.
Parsimmonious does not appear to be updating anymore, but there is so much Sims 3 (and a bit of Sims 4) content here, it's worth checking out.
Sims 4 DLC Sites
So I just really like this site. It is very much like the gallery except it's just for houses and it's somewhat juried i.e. submissions are reviewed. Fun to browse.
Sims 3 Sites
Most (if not all) of the really useful sims 3 mods are here. Home of Overwatch and MasterController as well as many, many more mods to make the most of your game.
Sim Worlds, gorgeous whole new worlds for sims 3 to play in plus residential and community lots of all kinds.
The Minecraft wiki is an indispensable aid to playing Minecraft. Everything you wanted to know is here inlcuding all the block types, recipes, etc.
Lots of Minecraft skins, texture packs and projects.
The Skindex has many pages of skins uploaded by the community. There is a also a skin editor which is quite nice.