US entertainer Jimmy Kimmel has been taking humorous pokes at Matt Damon for years, wrapping up every instalment of his daily talk show with the now infamous line “Sorry Matt Damon, we haven’t got time for you.” Finally the Hollywood actor bit back, making a music video with Sarah Silverman, the comedian and Kimmel’s girlfriend, in which she playfully sings “Jimmy, there’s something I’ve got to tell you - I’m f***ing Matt Damon.”

Now the comedy duel has spilled over on to YouTube, with over 15 million viewers visiting the site in the past week to watch Kimmel’s riposte, a video featuring a host of Hollywood stars from Brad Pitt to Robin Williams singing “Sorry Sarah, I’m f***ing Ben Affleck.”

But Damon, Pitt et al are just the latest in a long line of celebrities to foray into the world of online video. With the internet generation increasingly shunning the television in favour of the laptop, more and more celebrities are taking advantage of the quick, low cost production and global audience potential to air their sense of humour or even political views.

Justin Timberlake won an Emmy last year for D*** in a box, a hilarious take-off of an R&B video originally shown on Saturday Night Live. The clip went viral after being posted on YouTube and other video sites, as did another SNL sketch featuring the diminutive Natalie Portman as a gangster rapper. Both sketches spawned hundreds of amateur versions, with users the world over uploading their efforts to out-parody the parodies.

Will Ferrell has been one of the quickest to exploit the potential of online video, setting up www.funnyordie.com, a site where users upload their comedy creations. Celebrity offerings include Fergie of the Black Eyed Peas as a spelling bee contestant and musician John Mayer in a spoof of notorious viral video “Two girls one cup” – thankfully swapping the original faecal matter for strawberry frozen yoghurt. Comic actors Jimmy Fallon, Danny DeVito and Ed Helms all maintain their own channels on the site, while Ferrell himself regularly uploads low budget sketches including the phenomenally successful “Landlord,” in which the actor is threatened with eviction by his two-year-old but foul-mouthed landlord Pearl. The skit has since mutated into multiple versions of itself, including a clip run on Larry King Live in which Alec Baldwin’s apoplectic phone message to his young daughter was spliced together with Pearl’s tirade.

Not all celebrity sketches are quite so successful, however. Brooke Shield’s parody of a psycho-mum at a playground elicited only groans from most Funnyordie.com viewers, while Alanis Morrisette’s spoof cover of the Black Eyed Peas “My Humps” might have attracted over 12 million viewers on YouTube but was a little too poignant to be truly funny.

Online video has also proved an ideal vehicle for political campaigning, with stars such as Will.i.am and most recently Jack Nicholson wading into the Democratic primary race with ads for Barack Obama and Hillary Clinton respectively.

Will.i.am, frontman of the Black Eyed Peas, has produced two videos in support of the young Illinois senator, Yes We Can and We Are The Ones, both of which feature a string of appearances from celebrities including Jessica Alba, Ryan Philippe, Scarlett Johanssen, Herbie Hancock and Macy Gray. The first, in which celebrities sing along to an Obama speech, has received over 10 million hits on YouTube alone since it was posted a month ago.

Nicholson made use of his extensive filmography to produce his ad for Hillary Clinton, editing together clips of his most famous films – such as Batman, A Few Good Men and the Shining - to highlight the former first lady’s presidential qualities. A huge internet hit, it has already attracted over 1.2 million viewers on YouTube since Friday.

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