Technology executives like Microsoft Corp.'s Bill Gates or eBay Inc.'s Pierre Omidyar are known for opening their wallets for a variety of charitable causes. But in Silicon Valley, one philanthropic endeavor has recently been attracting corporate volunteer hours and funding from companies like Nvidia Corp.: Expanding access to healthy foods. “People are putting food in their mouths several times a day.” said Amie Frisch, executive director of San Jose nonprofit farm Veggielution.