Command-Not-Found is a program on Ubuntu that suggests a package to install if a command doesn't exist. Benji has recently got something similar working on OpenSUSE, so I decided to try to do it on Slackware :) My solution requires you to download MANIFEST.bz2 for your version of Slackware, and set $MANIFEST to the path of MANIFEST.bz2.

export PROMPT_COMMAND='
if [ $? -eq 127 ]; then
(
    history -a; 
    COMMAND=$(tail -n 1 $HISTFILE);
    LINE=$(bzgrep -m1 -n bin/$COMMAND$ $MANIFEST | cut -f1 -d:)
    if [ "$LINE" != "" ]; then
        PACKAGE=$(echo $SQL | bzcat $MANIFEST | head -n $LINE | tac | grep -m1 "Package" | rev | cut -f1,2 -d/ | rev)
    fi
    PACKAGE=${PACKAGE:="-"};
    if [ $PACKAGE != "-" ]; then 
        echo $COMMAND is not installed; Try installing $PACKAGE.
    fi;
)
fi'

Example:

fred@typhon:~$ kopete
-bash: kopete: command not found
kopete is not installed; Try installing kde/kdenetwork-3.5.8-i486-1.tgz.
fred@typhon:~$ foo
-bash: foo: command not found
fred@typhon:~$