Custom Content (CC) are custom items that are made from other players and added to the game, in The Sims 2 it includes create a sim/create a pet items, buy and build mode items, pose packs, neighborhoods decorations and mods. These are my bookmarked CC links.

Bigger websites & lists:

• Garden of Shadows
  Highly active CC creators forum
• Maxis Match CC sites list
  A list of sites that features maxis match CC
• Modthesims
  Old site with tons of stuff by a bunch of talented creators (also has a forum)
• Sims 2 Defaults
  A detailed list of defaulted CAS CC
• Sims 4 to 2
  CC finds simblr with The Sims 4 conversions
• TS1 to TS2 Conversion/Recreation Database (inactive)
  Livejournal with a list of The Sims recreations
• TS1 to TS2 Database
  Dreamwidth with a list of The Sims recreations
• TS3 Conversions (inactive)
  CC finds simblr with The Sims 3 conversions
• TS3 to TS2 Conversion Database
  Livejournal with a list of The Sims 3 conversions
• TS4 to TS2 (inactive)
  CC finds simblr with The Sims 4 conversions
• TS4 to TS2 Conversions (inactive)
  CC finds simblr with The Sims 4 conversions
• TSM to TS2 Conversion Database
  Livejournal with a list of The Sims Medieval conversions
• Werewolf mods list
  List of CC focusing on werewolves

Sites, Livejournal & Dreamwidth:

• Amovitam's Dream Town (inactive)
  Livejournal with CAS and Buy/Build mode CC
• Around The Sims 2
  Famous big website that focuses on thematic CC
• Cyjon's Sims 2 Mods 'n' Stuff (inactive)
  Website of a mod creator
• Imagination Figments
  Dreamwidth of a creator that has Buy/Build mode CC
• More Awesome Than You
  Creator's website of famous mods
• Mrs Mquve
  Dreamwidth with CAS and Buy/Build mode items that focuses on alien/alternative theme
• Simlogical
  Bunch of mods
• The Ninth Wave Sims
  Big website with tons of stuff
• Veranka (inactive)
  Livejournal featuring mostly Buy mode CC

Simblrs:

• Alien hair (inactive)
• Amidala Sims (inactive)
• Anitka Sims (inactive)
• Biffy sims (inactive)
• Chocolate Milk Bag (inactive)
• Crisp & Kerosene Sims
• DeeDee Sims
• Digital angels (inactive)
• Drool Worthy Sims
• Early Pleasantview
• Entropy Sims
• Esotheria Sims
• Funphumph
• Keoni-chan
• Lilith Sims
• Lilroisin
• MDP That's me
• Memento Sims (inactive)
• Midge The Tree
• Moonkist Sims (inactive)
• Plumbtales
• Poppet Sims
• Puccamichi (inactive)
• Rented Space
• Rudhira
• Sadepäivä's Sims 2
• SelenaQ13
• Serabiet
• Simbic
• Spectre Sims (inactive)
• Suratan zir
• Taylors Simblr (inactive)
• Ten Little Pandas
• The Sim Traveler
• Timeparadoxsims
• Vulrien Sims
• Xwhitepolar
• Y2 Sims

Other:

• Meetmetotheriver's clean templates
  Fixed/better versions of Maxis neighborhoods
• Meetmetotheriver's Clean Premade EA/Maxis Neighbourhoods
  Fixed/better versions of Maxis sub-hoods
• The Sims 2 Primer: Avoiding Game and Neighborhood Corruption [not really CC but essential to play The Sims 2 if you get attached to your neighborhoods and sims]