Path To Freedom is the web home of the Dervaes family. It's an inspiring site with lots of useful information and examples of urban homesteading. Their recent blog post Seasonal Eating sets out to provide resources to help people in the US to eat seasonally.
Slow Food Sydney have published a guide for consumers in the Sydney bio region that not only shows what's in season, it also indicates whether the items can be sourced from the remaining farms in our local bio region. It's a small self published booklet that deserves support. Covering Fruit, Vegetables, Meat and Fish grown no more than 160km from the CBD, it's $10 and available from Slow Food volunteers at Farmers Markets and a small selection of shops. Check out this site for more info.
Another useful online guide to fruit and vegetables in season around Australia can be found here.