I finished working on SlackBuilds for the VirtualBox Guest Additions. They are available as of today from my website (http://www.liwjatan.at/slackware/system.php5
). (one for the kernel modules and one for the userspace part)
For all who do not know what those are about, the guest additions allow using shared folders (mounting folders of your host system inside the virtual machine), shared clipboard (copy from the vm to the host and vice versa), mousepointer integration (you can use the mousepointer inside the vm without capturing it in that window), screen resolution independency (you can resize the vm-window to any size and xorg will automatically adjust the resolution), time sync with the host and probably something else I missed
Have fun with those. If there are any questions or problems with the script, please report them here or per mail directly to me. My mail-address is included in the SlackBuilds directly and also in the .info files.
Edit 20081202: Updated for new location