You can refer to the individual threads in this forum for specific discussion on the listed sites.

Bookins (US only) · (thread)

BookMooch (International) · (thread)

Books Free Swap (US only) · (thread)

FrugalReader (US only) · (thread) NO LONGER ACTIVE

PaperBackSwap (US only) · (thread)

PoshPoints (US only, trade anything) · (thread) NO LONGER ACTIVE

Swap.com (formerly SwapTree) (US only, books, music, movies, games) · (thread)

TitleTrader (International, also CDs and DVDs) · (thread)