I'm making informative MP3s on Unix, VMware and SAN topics. They'll be available on a pay-per-download basis once they're ready.
I sometimes fix up and sell used Lenovo ThinkPad X and T series laptops. These give people access to the same quality hardware big businesses use, for only five or six hundred bucks.
As of late 2015, the best bargains are those based on Intel Sandy Bridge or Ivy Bridge processors, because they're fast enough to do everything people usually use computers for, yet old enough to be cheap and have great Unix compatibility. This includes these models: X220, X230, T420, T430, T520 and T530.
Though I usually recommend Macintosh computers, they hold too much value to be good bargains used. For instance, late 2012 Mac Minis are currently $500 or $600 on eBay, which is barely less than the current models. And since Apple makes both the hardware and the operating system, there's no compatibility advantage to buying older Macs; the new ones work just as well.