UK : Amazon.co.uk  |  Blackwells  |  The Internet Bookshop   |  The Book Depository (offers free delivery worldwide on all books)

North America : Amazon.com  |  Barnes and Noble

France : Chapitre.com  |  Amazon.fr |  Fnac  |   Bouquinerie L'Ex-Libris  |  La Procure (for religious books, especially)

Germany : Amazon.de

Australia : Bookworm  |  Dymocks

Korea : Kyobo (direct link to Search), Yongpung, Yes24, AladdinUS  (These sites are all in Korean but AladdinUS also has an English-language page and services based in the US).

To buy books, videos and DVDs etc related to Korea in English (or with English subtitles) the Seoul Selection online bookstore is a major resource. They welcome information about new publications. They also publish a number of titles themselves. Books about Korea can also be purchased from  Koryo Books located in the US.  If you are in Seoul, Seoul Selection is also a place you can visit in person to browse or drink coffee in since their bookstore is also a cafe.

For Korean software (and indeed software in any language) see World Language Resources' Korean list.

To buy foreign books inside Korea, Interpark is recommended, though I have never used it.

Bookfinder.com  is a very good resource for finding out -of -print books, but perhaps Alibris is better...

Abebooks.com (Advanced Book Exchange) allows you to search the stocks of many independent bookellers for out-of-print and rare books.