Finger limes are one of those foods that bring out the little kid in even the most staid grown-up. I dare you not to squeal when you cut one open, squeeze, and see all that bubbly goodness emerge.
First discovered in the wilds of Australia, finger limes (aka Citrus australasica) are small—about three inches in length—and are now grown in small batches in the U.S. They also go by the nicknames citrus caviar or caviar limes. Blogger Food Librarian affectionately calls them "lime boba."
If you happen to see any at your local farmer's market, be sure to grab them. Their taste and texture are amazing: small bursts of fresh lime-ness that explode on your tongue. I especially love using them in desserts: scatter them across the top of a Key lime tart to add extra visual interest and flavor or use them to provide a nice citrus counterpoint to a blackberry sorbet. They also work great in cocktails, on very sweet fruit for contrast, in salads, and with certain heartier types of seafood.
Freida Produce has a great video that tells you all about finger limes and how to use them in your cooking.
If you're interested in experiencing finger limes for yourself, they're available from July through January. Ask around at your local farmer's market, or you can order them online here or here. Warning: they're not cheap, but they are worth it!