Uh-oh -- looks like FrontierVille is targeting collection hounds by stocking the in-game market with Mystery Crates.
The mystery crates can be found in the 'Special' section of the market and cost 25 Horseshoes. You'll gain 10 XP by purchasing one of these bad boys, and you'll get one of six collectible items. What those are -- well, I don't know just yet. I'm too cheap to shell out for these crates, so will have to do some research to find out what's lurking in that wooden crate.
What did you find inside your FrontierVille Mystery Box? If you have pictures, send them to firstname.lastname@example.org, and I'll add them here.