What plants have volunteered in your garden? I'm thinking edibles, but maybe you have flowers or weeds or another notable plant that has made your garden its home? Include your zone and any other helpful information...
Here's my offering:
I have always loved plants that reproduced themselves. Kale and arugula come to mind; Italian parsley, leeks, lettuce, edible pod radish, currant tomatoes, even one year tomatilloes! The oddest was the carrots. I never had luck with self-sown carrots in the garden, except for making pretty flowers. But the seeds ended up in the compost, and the compost eventually was used in the pots we placed on the roof to grow jalapenos. The carrotseed would sprout and grow long, fat, worm-free carrots. On the roof! which made even tomatoes and eggplants wilt and die a horrific death.
Now, our home in zone 7 is too near a field with a lot of Queen Anne's Lace (same species) and the volunteers nearly always have roots like those.