We were browsing our new local (year-round) farmer's market in Ballard a few weekends ago, when we stumbled upon a mushroom vendor with a grow-your-own kit! The vendor was Sno Valley Mushrooms and they sell kits that you can order online if you're not local to the area!
We tried the Shiitake kit. It's really easy to use and the mushrooms grow quickly! The kit comes looking like a bread loaf, covered in mycelia. You keep the loaf in a warm, sunny, moist place (we used a plastic bag to cover it like a green-house - it worked pretty well). Here are some photos of their growth process:
After the first harvest I was able to fill a full pasta-bowl full of the mushrooms - yum!