Bookswapping

• Bookmooch
• PaperBackSwap
• Readers United
• FrugalReader
• Title Trader
• Bookins
• WhatsOnMyBookshelf.
• Novel Action
• ReadItSwapIt

Bookhopping

• BookCrossing
• America’s BookShelf
• BookHopper

Really free books

• Library
• Friends, family, neighbors, classmates
• Free ebooks include Project Gutenberg and Free-ebooks.net.

Other options

• BooksFree
• Zunafish
• Thrift shops.
• Swapbooks
• half.com
• LibraryThing

via zenhabits