closet shelving rail ("standards") systems

• ClosetMaid ShelfTrack — the most popular and one that's been around the longest
• RubberMaid — also fairly popular
• John Sterling — third-place in popularity
• a few other random ones:
  • Elfa from Container Store
  • ISS Designs
  • Knape & Vogt
• there are a lot of companies that produce generic standards under the name slotted strut channel
  • "Superstrut vs. Unistrut"
• information about intercompatibility