I'm using my laptop right now. It's mostly stationary though, since it's roughly the size and weight of a small attaché case belonging to a travelling lead weight salesman. It's not old, we have a very old laptop as well that's more the size and weight of a slab of depleted uranium, but my laptop's just heavy. I dunno why.
My dream, though, is Other; building (or rather, having someone build, as I know bugger all about building computers
) a supercomputer from cheap old parts, linking them all together into one system. I just checked Intel.com, and 2.2GHz CPUs are selling for around $64, so I'm guessing older processors will be a lot cheaper, and by the time I actually have the ability to make this dream come true I can probably get 4GHz CPUs for $5 apiece or something. Buy 20. Link into four virtual cores. Voilà, 20GHz quad core CPU for $100. I know it's probably a lot more complicated than that, but hey, a man can dream right?