I still have the 16k RAM pack but gave the ZX81 away a few years ago to a colleague who was mourning the loss of his own.
Hee-hee, Ram Pack Wobble! - I remember that well and how it was solved by a strategically placed piece of Blu-tac!
My first computer was a Sinclair ZX Spectrum, 48k model. Spent the whole summer learning ZX basic on that thing and wrote a few games, including a simple platformer that even had a level designer (but no ability to save levels). Not bad for a 48k of RAM and a novice programmer!