Stand up comedy is now seen by programmers as televisual Polyfilla. Got an awkward gap in the schedules? Plug it with a half-hour showcase. Need a game-show host? Reach for a satirical monologist. Consequently, the Edinburgh Fringe is no longer the place where you can see tomorrow's TV stars today. In our post-Channel 5 world, in which you can't tell your gran a knock-knock joke without being offered your own late-night talk show, the place to see tomorrow's TV stars today is on TV.

For those festival-goers left wondering why they should pay pounds 9 to see a comedian in a muggy auditorium when they can see him in their own living rooms for nothing (to attend all of the shows I review below would cost you pounds 59.50), one answer is that television is not, lest we forget, a stand- up's natural habitat. If you know Greg Proops from nowhere but Whose Line Is It Anyway? and Space Cadets, you'll have him down as a slimy drip of sarcasm with an annoying habit of saying "obvioso" instead of "obvious". If you see him live, you'll find that television has not deceived you. But you'll also discover Proops's dark secret: beneath the suit and the specs lurks a rather endearing, humane comedian with a liberal political agenda.

Space Cadets viewers will be aware that Proops's colleague, Bill Bailey, looks like a pop-eyed Tolkien wizard, or the late Willie Rushton after a week in Glastonbury. They may be less aware of how naturally, effortlessly hilarious he can be, even when shambling through some frazzled theory about why the Sponge Age never caught on as the successor to the Stone Age. Nor will couch potatoes have sampled his expertise on piano and guitar. Reminiscing about Seventies TV shows may be scraping the bottom of the comedy barrel, but Bailey's musical impersonations allow him to cook up those scrapings into something tasty: his recreation of incidental music from The Professionals is somehow side-splitting whether you have a single memory of the programme or not. There may be stand-ups who are more incisive, and indeed better dressed, but if comedic ability comes down to the simple matter of inducing physical pain via excess laughter, Bailey is the cream.

Sean Hughes (file under: a lot funnier than he is on television) is still harping on about being a lonely misanthrope, a subject which would have us rushing towards the file exits by now if he weren't a consummate harper. Loose-limbed and confident, he has developed his stand-up to the point at which it blurs into a one-man play, complete with slides, music and props. And no, that's not a euphemistic way of saying that there aren't enough jokes.

The dramatic crux is whether or not Hughes should commit to a mystery woman: "Let's call her X, because no doubt she'll soon become one." If he stays single, he'll have to use a basket in supermarkets, because the assistants can sense that he doesn't need a trolley. But in a relationship, life could be even worse: "Married couples only ever leave the house for emergencies - shopping and shelving."

There's more to the show than bachelor-boy dilemmas. Hughes casts some piercing glances at Catholicism, pornography and drugs, and so casually and freshly that one feels embarrassed for certain other comedians, who wet themselves in self-congratulation when they get within a royal mile of anything as controversial.

I say "certain other comedians", I mean Scott Capurro and David Baddiel. Capurro is gay. If you think that's a taboo-busting, side-splitting statement, then you'll be impressed by his show. If you don't, you won't. David Baddiel makes a pertinent case study, in that he explains why it is that Fringe comedians are so keen to get their faces on television. For Baddiel is no funnier than the second-funniest of your mates in the pub, and yet the goodwill garnered by his TV persona enables him to pass off a fragmentary hour of outmoded, complacent chat as a stand-up show. He has misread two entertainment maxims. First, comedy may be truth, but, whatever Keats may tell you, just because something is true doesn't necessarily mean that it's comedy. Second, a judiciously placed swear-word can give a punchline extra punch, but it helps to have a punchline in the first place.

Finally, there are still those acts whom you don't see much of on television. The unique, mind-boggling stand-up of Johnny Vegas, a Perrier award nominee, has made him the overnight sensation of the Fringe. I'm not sure if his art counts as stand-up, however, not least because he spends the second half of his show sitting at a potter's wheel. Maybe it's character comedy, but there's no hint of who the man behind the character might be. We're confronted by a mountainous Northern sot who rants about how his career at Butlins was ruined by scandal, only for his sanity to be saved by a love of pottery: "Teapots got me out of bed in the morning." He punctuates this autobiography with showbiz similes ("Comedy's like marrying your cousin: you can listen to your critics or you can listen to your heart"), makes a jug out of clay moistened with lager, and finishes with a sing-song. Potter's wheel aside, what makes this show unique is the rage with which Vegas berates his audience at the start of his show, and the warm glow we nonetheless take away at the end.

Rich Hall is almost worth seeing for his crackling voice alone: imagine the Al Pacino character in Scent of a Woman as played by Gonzo from The Muppets. A sceptical, seasoned veteran one minute and a twinkle-eyed mischief- maker the next, Hall can joke about everything from the Hubble telescope to Scottish devolution, before, in accordance with the statutes of the Male Stand-up Comics' Guild, he comments that one gender has a thousand more emotions than the other: men get mad, women get "confused and vaguely Edwardian".

Scott Capurro and Greg Proops, the Pleasance (0131 556 6550), until Sat. Rich Hall and Johnny Vegas, the Gilded Balloon (0131 226 2151), until Sat. Bill Bailey and Sean Hughes, George Sq Theatre (0131 650 2001), until Fri.