Sauvignon Blanc - SB is fairly acidic which means refreshing. A cold Sauvignon Blanc can revitalize on a hot day. There are lots of SBs available from all parts of the world and can be found at very reasonable prices compared to Chardonnay.
Example: Hanna Russian River Sauvignon Blanc
Unoaked Chardonnay - These are usually lighter, crisper and cheaper than the traditional Chard.
Example: Iron Horse Russian River Unoaked Chardonnay
Rosé - Before you say, "Eeeewwww" I'm talking about drier wines made from quality grapes, not the five dollar stuff. A good Rosé is light and easy-drinking. How do you know a quality, lower sugar Rosé for a cheap, sweet one? It's probably best to check online data for the wine first--look for the grapes in the blend and the residual sugar.
Example: Scherrer Sonoma County Rosé
Riesling & Gewurztraminer - I lumped these two together as their description is similar in that as an off-dry wine they are also refreshing like a Sauvignon Blanc, but with a touch of sweetness to balance the acidity. If you get a wine that's too sweet it may seem a bit heavy for a summer sipper.
Example: Smith-Madrone Spring Mountain Riesling
Sparkling - A good off-dry or dry bubbly is refreshing and pairs well with lots of meals including spicy or salty foods. Best for summer are Brut, Blanc de Blanc and Rosés.
Example: Gloria Ferrer Brut Rosé
Other Whites - Everyone knows about Pinot Gris, but maybe not Chenin Blanc or Semillon.
Example: Dry Creek Vineyards Clarksburg Chenin Blanc
Grenache - A lighter Rhone red variety that is often used for blending, but seems to be catching on in California in it's own right. It can be an easy-drinking red wine that will go with most summer foods. Only issue is that it has a propensity for high alcohol levels which isn't so good for a food wine or a summer sipper so pay attention to the label. Also look for Grenache blends that are typically with Syrah and Mouvedre. Note that wines which are primarily Syrah will be a heavier wine so a Grenache / Mouvedre blend might be best for a warm weather wine.
Example: Mounts Family Dry Creek Grenache
Other Red Wines - Go lighter. This means if you're having beef and would be thinking Cabernet go with Merlot, Zinfandel, or even Pinot Noir. The lighter-flavored beef dishes, like fillets and tenderloins and even burgers, can work with Pinot or Zin. Also look at alcohol levels as higher levels can mean a heavier wine. A hot summer night is not the time for a 16% alcohol Zin!
Example: Nalle Dry Creek Zinfandel
Be sure any red is at a cool room temp. This can sometimes be difficult in hot weather. There's nothing worse than a warm bottle of Cabernet on a hot day. Store the wine in the coolest part of your home. Put the bottle in the fridge for 20 minutes before dinner if necessary.
Or you can always go this way ...