At a Seattle conference on podcasts recently, the new wave of audio content producers on the web were greeted with postcards that said: "Podcasters! Are you here to join the revolution - or the next NBC? Prepared to shape this new media in your own image? Or eager to conform to theirs?"

You would have had to have your computer on silent in order to miss the "podcasting revolution". Podcasts started out as just a cool geek way to post audio files online.

This could be anything from your local band's first anthem to your mother reciting her favourite recipes; a podcast could be sent from tech conferences or book readings recording the speeches of the famous; they could be entirely unoriginal content (the first "podcast" was just a Grateful Dead song that had been converted into MP3 format).

For a while it seemed as though podcasts - the word combines "broadcast" with "iPod", following the success of Apple's MP3 music player - would bring back the kind of energy that we had seen in the early days of blogging. The net was full of reports on "citizen podcasters", commentators spoke of a return to purity and sneakily hoped for yet another battle between old and new media.

Initially, podcasting, like blogging, delivered on most of these fronts: if you wanted cutting edge tech news, the best of non-label world music, the sharpest counter-culture opinions, they were all there, on podcasts. The latest in a series of brave new worlds was looking a lot like a cross between the Soviet-era samizdat publications and ham radio.

It's a little different today. BBC Radio is a leading podcaster, as is CNN, Fox, Al-Jazeera and every other key media group. Though podcasts can be random-searched through neat tools like Podzinger (www.podzinger.com) or filter blogs like Podcaster Alley (www.podcastalley.com), the medium has now gone mainstream. The two sites most frequently searched by podcast newbies and enthusiasts are Apple's iTunes and Yahoo!'s

Podcast search (www.apple.com/podcasting, www.podcasts.yahoo.com).

And while they're far more open to the new, the unknown and the quirky than most mainstream media sources, the most popular podcast list is beginning to harden, in the same way that blogging did.

Yahoo!'s top podcasts are increasingly produced by professionals, from NPR, or Slate, or GameSpot. Apple's list includes podcasts from ABC News and ESPN. The top music podcasts are also chiefly from existing hit radio shows, or record labels.

As podcasts built up a dedicated audience, the movement back to the mainstream became inevitable. Listeners wanted quality, with no rough edges, and wanted the familiar rather than the oddball. But the interesting thing about podcasts and blogs is not that they have mainstreamed to a great extent; it's that they have managed to retain an edge of independence.

Godcasting and "bodcasting" - religion-oriented podcasts and porn/adult mainstream podcasts - are still the domain of amateur zealots, though both are now developing into mini-industries.

Despite the dominance of the NBCs and the CNNs, though, "oddcasting" still has a very firm place. Popular recent podcasts have included DIY viewer museum guides including one where the viewer expressed his opinion of great art via orgasmic moans or disappointed groans, finetuned and very specific travel guide podcasts, which can sometimes zero in on just one monument or three restaurants in a neighbourhood.

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