We used a service recently that performed well, and saves a bundle over the typical AT&T $200 bill for an hour-long 4-5 way domestic conference call using an 800 number. The one we used was www.freeconferencecall.com; essentially, each party pays for its own long distance charge. For 800 conference calls, it's $.14 to $.18 per minute for each party; that's still a small fraction of what AT&T charges. We've only used it once, but had no problems at all, and sound quality was fine.

There's a competitive service at www.freeconferencecalls.com (note the "s" at the end on that one) that seems to provide the same service for $.10/minute/caller for 800 calls. Not bad; we haven't used them, though.