If you feel comfortable and safe telling your partner(s) yourself, by all means go aheadperhaps over the phone or in an email. You can even set up an anonymous email account (just Google the phrase to find options) to alert your partner(s). But if you don't want to tell them yourself, you do have other choices.
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.