I've been using Prgmr's $36 package for quite a while now and it's worked out great. It's half the price of Linode, but you have to know what you're doing.
I am mainly interested in providers with EU presence because of the latency which is a reason I've never tried prgmr.com. Are you able to install Slackware there?
Would be nice to have a directory of Slackware friendly providers...
"Any OS paravirtualized to work under Xen should work. We will install Linux (Debian 5.0.3, ubuntu 10.04, or CentOS 5.5) and we provide instructions so you can install your own NetBSD."