Your local health department may contact you if you are diagnosed with an STD to make sure you're getting proper treatment. If that happens, the caller will probably offer to notify your partner(s) for you, keeping you anonymous, or to be with you when you tell your partner(s). If your health department doesn't offer this service, you can ask them for it.
A website called inSPOT, run by the sexual health nonprofit Internet Sexuality Information Services (ISIS), allows people in eight major U.S. cities (San Francisco; Chicago; Los Angeles; New York City; Philadelphia; Portland, Oregon; Seattle; and Washington, D.C.) and eight states (California, Colorado, Florida, Idaho, Indiana, Massachusetts, Minnesota, and Wisconsin), as well as in Ottawa and Toronto in Canada, to alert sex partners anonymously of an STD diagnosis and urge them to be tested. It also shows them where to find more information on STDs. The website has plans to expand into many more states and cities soon.