Oranges: Squish them a little; if they have some give, it means their rind is thick and they've had time to mature. Or, as with grapefruits, weigh them in your hand; if they're comparatively heavy, they're nice and juicy. (Note: contrary to popular belief, the orangest oranges are not necessarily ripe; it only means they were grown in colder temperatures.)
Peaches: First smell the peach to make sure it smells good; if you can't smell anything, it's not ripe. Then feel it to make sure it is tender (but don't push too hard). Taking these simple steps should save you the disappointment of eating a sandy, tasteless peach.
Cherries: Look for deeply colored, firm (but not hard) cherries with green stems (as opposed to dark, which will be old).
Watermelon: Look at the discoloration, where the melon has rested on the ground. If it's a big, broad discoloration (whitish-yellow) along the bottom of the melon, it's been sitting in the sun a lot, ripening, and it's perfect. If it's just a small spot of discoloration, leave the fruit for someone else.
Pineapple: The leaves of the best pineapples, when bent over on themselves, should snap at some point, rather than just bend. Overripe pineapples will smell sour.
Grapes: The best, crisp grapes will be lighter than those that are squishy and old; pinch a few on the bunch to see if they are firm. For ripeness, look for flecks of tough, brown skin, which are signals that the grapes have had good time to ripen in the sun.
A note on strawberries and tomatoes: The best strawberries and tomatoes are those that are freshly grown. Check around with your friends to see if any of them have plants, or look for roadside stands and farmers' markets that sell them. Visit fruitstands.com to help you find local farmers' markets or fruit stands.