A different (but still very time-consuming) hobby

[DutchMarco]

This is what I waste most of my time on - wielding hot metal stick and melting tin to things that'll carry lethal voltages. Great fun! and that's before you can whip out the power tools (whip them out gently and slowly, esp. if others are around) to build the wooden box these'll fit in.
It's not the kind of Firefly that'll play such a big role in a tv-series like Serenity: http://picasaweb.google.com/magicmojo1/MojoWatt#
Find other (already finished or abandoned) projects on my website:http://marco.ooijer.com
Oh, and if you have a good relationship with your neighbours this is a brilliant path to changing that. ("If you'll build it, they'll await peacefully and not yet come." "If you'll play it, they most certainly will do just that.")

[Renal Shutdown]

Hm, almost similar to a 'hobby' I was tempted to start a few years ago.  Though, instead of amps, I was planning on making my own effects pedals/racks, and also build the effects directly into custom guitars, too.

I never got around to bothering in the end, as I quit my band and pretty much took a long break from guitar/bass entirely.  I'm playing a pretty lousy/damaged acoustic guitar at the moment, and I've re-found my enjoyment for it, so who knows I may venture into electric again soon.  Which means I may come to you for advice in the distant future.

[TerranRich]

I've gotten into video editing, audio editing, DVD menu authoring, and this thing called AGS.

Your hobby kicks my hobbies' ass, of course.