The easiest thing is to serve them raw with butter and salt. From the moment you yank them out of the soil to having them in your mouth, we're talking 3 minutes tops. They'll practically be still pulsing with life. They're also nice sliced ultra-thin (have you gotten a Benriner mandoline yet?)
Or, if you're feeling ambitious, spend some time braising, see Faith's popular post on Recipe: Spring Radishes Braised with Shallots and Vinegar.
Even if you don't have a garden, most radishes fetched from a farmers' market will have been plucked just hours prior, so you'll be treated to something incredibly fresh. Welcome to spring!
Look at this gorgeous harvest of French Breakfast Radishes from the archives of GenreCookshop.com
Some other radish bits:
Recipe: Spring Radishes Braised with Shallots and Vinegar
Simple Pleasures: Radishes with Butter and Sea Salt
Braised Radishes Again This Time with Rice Vinegar and Mint
In Season: French Breakfast Radishes
Look! Radishes and Buttered Bread in Bon Appétit