The package for CentOS 5.5 is provided by the RPMForge
repository. The tool puttygen comes along with other tools from the package. I cannot find it for OpenSuSE 11.3, but if you need help for compilation feel free to ask.
Trying to compile on OpenSuSE you might have problems with missing GTK libraries. However you can still compile the command line tools (like puttygen) that don't require graphic libraries. This should be as simple as:
# tar xf putty-0.60.tar.gz
# cd putty-0.60
# cd unix
# make install
Hope this helps.