JEFF JARVIS REVIEWS video hosting services.
For people having problems with uneven sound levels, I recommend a good compressor. (No need to shell out to buy a great compressor for this stuff; the beauty of the Web is that it lets you get away with a lot). I use this one. I’ve noticed on our podcasts, though, that the narrow frequency range of telephone calls makes them sound less loud even when they’re at the same level as voices in the studio. Getting closer to the mike helps, too.
UPDATE: Various readers — rather vehemently — recommend this budget compressor over the Alesis I link above. I’ve never used it. The Alesis isn’t something I’d rush to use in a musical application, but it’s entirely adequate for the purposes — basically, a brickwall limiter — that I use it for in podcasting.