Custom Content (CC) are custom items that are made from other players and added to the game, in The Sims it includes custom skin, heads, objects, hacked objects and mods. These are my bookmarked CC links.

Lists and Archives:

• Sims Angelfire Sites
  2GB of archived CC from over 200 Angelfire sites
• Treasured Twice
  Collection of Yahoo Groups CC's that were saved before it's deletion
• Juniper Sun
  Archived buy/build mode CC and housing lots from this creator
• Sims of The World's Active Sites List (inactive site)
  Outdated list of "active" The Sims CC sites
• Sims 1 CC
  A tumblr with a bunch of archived CC from an old simmer

Mostly CAS:

• Abercrombie and sim (inactive site)
• Blackbird's Lullaby (inactive site)
• ST Fashion
• Watersim YMDesigns

Mostly Buy/Build mode objects:

• A Little White Magic
  Lots of hacked items
• Boudoir Blanc
  Ton of decor items including fake/mannequin sims
• Classic Sims
  Conversions of TS4/TSO and Maxis match stuff
• Ferndale Sims
  Tons of stuff, includes TS3 conversions and Maxis match stuff
• Grumpy Sims Site
• Ingrid Studio
• Madoria's World of Sims 1 Downloads
  Tons of stuff, includes Maxis match stuff and hacked objects
• Mirrored Sims
• Nocturnal Market
• Nyx's Stuff
• Oleana's Clutter Factory
• Persimmon Grove (inactive site)
  Nature themed items and custom lots
• Poetica Place
• QB Sims
• Sims 1 Mods by Corylea
  Tons of hacked items
• Sims Versailles
  A project to recreate everything from The Palace of Versailles in CC
• SpaceCadet's Sims
• Stray Pixels
  Walls, floors, paintings and fake sims
• The Sims Emporium
• XM Sims (inactive)
• Yuripuda

Both CAS, Buy/Build mode and hacked objects:

• Around the sims 1
  Its main feature is their themed sets
• Awesome Expression
• Barairo no jinsei/La vie en Rose Sims1 Works
  Has pet skins
• Eph's Den of Many Sims
  Fairly big site with tons of stuff
• Gnohmon Sims 1 goodies (inactive site)
  Hacked objects
• Magnolia Sims
  Themed historical and fantasy sets
• OddSim (inactive site)
  Conversion of TSO items and NPC's skins
• Nostalgia Sims Things
• Parsimonious
  Fairly big site with tons of stuff
• Russian Vacation Sims
• Simlogical
  Hacked objects
• Somesimthings
  Also features hacked objects
• The Secret Society of Woobsha
  Has a custom neighborhood map, hacked objects and more cool stuff
• Vivacious

Other:

• Sim Echoes
  Archive of premade Maxis' houses