My current mountain bike is a Gary Fisher Hoo Koo E Koo. Its a 26 inch multi speed with Shimano/Avid caliper brakes, Shimano gears and a Rock Shox suspension fork.
I got it off Craigslist a while back for $180. Its been a fantastic bike thus far. Its relatively light since it has small tubing yet strong since it is chromoly (a mix of steels). It came with road tires which I quickly swapped out for the lightest dirt tires I could find, Maxxis Larsen TT.
I haven't changed much on it. Upgraded the brake levers cause they didn't fit my hands very well and brake pads as they were terribly worn. Replaced the grips once they started slipping and getting slimy. Otherwise, nothing has broken or failed since I got it.
To accessorize, I got a water bottle and holder, mini tire pump and a saddle bag (small pouch that straps to the bottom of the seat to hold tools and a spare tube). That's really all I need, simple and solid and a pleasure to grab and go without the new bike panic (being afraid to ride it from fear of dirt or damage).
There's a few things I'd like to upgrade still like the pedals, seat and possibly the rear hub. In the meantime, I'll keep riding and enjoying!