In fall of 2016 (the start of my senior year of university), I bought a Thinkpad X40 for $40. After putting a 32GB CompactFlash card in, installing Debian, and increasing the RAM to 1GB, I started using it as my primary computer. (I didn't have to do that, I had a perfectly-good Thinkpad T430, but I wanted to give the X40 a good try.)
The ThinkPad X series is a line of notebook computers and convertible tablets originally produced by IBM and now marketed by Lenovo. IBM announced the ThinkPad X series (initially the X20) in September 2000 with the intention of providing “workers on the move with a better experience in extra-thin and extra-light mobile computing.” The ThinkPad X series replaced both the 240 and 570.
(For the record, my X40 had an ultra-low voltage Pentium M Dothan running at 1.1 GHz.)
It worked great for me, and did everything I needed or wanted it to do. I actually didn't use my T430 at all that school year, because the X40 worked fine for everything I was doing.
Unsurprisingly, it was fine for doing assignments in Python and C. It wasn't perfect with DrRacket -- I had to close and reopen every half hour as memory usage increased -- but it worked, and then I realized I could just use `gracket` or `racket`. I did a bunch of PDF/image editing in GIMP to clean up the lousy scans one of my professors gave us every week to read from. I wrote papers and presentations in LibreOffice. I killed time on reddit. I taught myself how to do CAD in SolveSpace. Most impressively, I ran CompuCell (a voxel-based biochemsitry simulator) in a windows virtual machine, fully interactively and only slightly slower than my classmates' computers.
I loved that computer. The battery would easily through one 90-minute class, and I could stretch it through two if I knew I wouldn't have an outlet. The keyboard was perfectly sized for me, and very comfy to type on. The trackpoint worked and felt great. (People didn't ask to borrow the laptop, since it din't have a trackpad.) The display fitted a satisfactory amount of stuff on it. It had real USB ports on it, just as people started having laptops without any USB-A ports at all. Using a laptop that was just able to do everything I needed it to do, I felt a lot more like I was working with the laptop, rather than just on it. (Perhaps this is a similar feeling to what people get when driving Maxda Miatas or BMW M3s -- working with the machine to get the most out of it.) And, in my eyes, it looked great -- a platonic ideal of a laptop.
Unfortunately, the X40 has a fatal flaw -- the southbridge tends to die. When that happened, I spent another $30 on a new motherboard. That also died, right around the end of the school year, in late spring of 2017. I decided I didn't want to to through that again, and I gave away the husk of my favorite computer ever -- my own 'oldest viable laptop'.