People smell sometimes. That's just a fact of life. And often these people don't know they smell. That's a very unfortunate fact of life.
The problem is that you want to tell them, but you feel like that is socially unacceptable. It will be awkward, and you risk offending the person. But in the long term (and even the short term), you are doing this person a favor. It's much better than keeping quiet - everyone else knows they smell!
But you don't say anything. You're too embarrassed. You don't want to be mean (when in fact your silence is meaner).
All Too Flat to the rescue!!! Introducing the ATF Stink-o-Grams. This is a completely anonymous email application that you can use as a free service to tell your friends that they smell. The emails will be addressed to the recipient and explain very courteously the problem and offer proposed solutions. There will be no evidence of you or your name whatsoever.

New! Due to the overwhelming success of ATF Stink-o-Grams, we have listened to your suggestions and now offer a variety of social inadequecies that you can email your friends about.