apt-show-versions is a Debian tool that lists available package versions with distributions. This is really useful if you have a mixed stable/testing environment and want to list all packages which are from testing and can be upgraded in testing.

This package is not installed automatically so you have to install it your self

apt-get install apt-show-versions

An example output from this command

groff-base/etch uptodate
notification-daemon/etch uptodate 0.3.5-1+b1
libavahi-common3/etch uptodate 0.6.16-3etch1
esound-common/etch uptodate 0.2.36-3
libgnome2-0/etch uptodate 2.16.0-2
m4/etch uptodate 1.4.8-2
libkpathsea4/etch uptodate 3.0-30

The -u option displays a list of upgradeable packages

# apt-show-versions -u    
     # apt-show-versions -u
linux-image-2.6.18-6-686/etch upgradeable from 2.6.18.dfsg.1-17etch1 to 2.6.18.dfsg.1-18etch1