Alien is a computer program that converts different Linux package distribution file formats to Debian. It supports conversion between Linux Standard Base, RPM, deb, Stampede (.slp) and Slackware (tgz) packages.
To convert a .rpm into a .deb simple run:
alien --to-deb /path/to/file.rpm
And vice-versa:
alien --to-rpm /path/to/file.deb