you could use sftp, wich is a ftp over ssh connection (so your sshd must be running on the server side) or you could use NFS (Network File System), with wich you can create 'shares'. other possibilities are through ftp or even http... it's best to look at wich tools are available on the machines. if you need just one file, sftp is easy (sshd runs standard on the slack at least) but for many files to transfer you should go for another method. for NFS you need kernel support (client && server side). slackware comes with an FTP server, so that's a good option too, I guess knoppix has some ftp client ready...
.-=~ iluvatar ~=-.