By Twain (for Win32)
JTwain
http://asprise.com/product/jtwain/index.php
Morena
http://www.gnome.sk/Twain/jtp.html
By SANE (for Unix-like platforms)
JSANE
http://asprise.com/product/jsane/index.php
By JMF
(some sample codes)
http://ltu164.ltu.edu/itseng/version.htm
Others
TimCam
http://timcam.sourceforge.net/