The problem with swapping movies, TV programmes, or songs for free over the net is that a lot of the content shared is often copyrighted - which makes sharing it illegal.
File-sharing systems are now more wary of copyright actions
And that is where many file-sharing systems have come unstuck.
In 2005, 28 entertainment companies sued the makers of the file-sharing program Grokster.
The Supreme Court decided that Grokster encouraged illegal file-sharing, and it was therefore liable when its users ended up trading copyright works.
It was an important ruling, potentially opening the floodgates to more action against all file-sharing systems.
Faced with enormous fines, the makers decided to close Grokster down.
Keen to step out of the firing line of the law, others followed suit, and vanished of their own accord.
Some remain, but now kindly ask that their product is not used in territories where the courts have already ruled against them.
BitTorrent is a lot more difficult for the authorities to shut down. It is not just one program - it is simply a method of sharing files which is used by many programs freely available online.
There are no servers to shut down. There is no one company to sue. There is no target to hit.
This has not stopped the authorities taking aim at services which are indirectly associated with BitTorrent. For example, many search engines now help users to find torrents to download and share.
One such portal is being sued by the US film industry for assisting in the sharing of copyrighted content.
The portal says it does not deal with illegal file-sharing, but a quick browse at the latest torrents shows otherwise.
If ever you needed proof that there is a lot of illegal file-sharing going on, take a look at what is available at these portals.
The enormous download capabilities mean you do not just download one song. MP3 copies of the entire music chart are available in one torrent.
These are the places where TV programmes are shared, as soon as they have been shown on TV.
Just a few hours after broadcast, the BBC's Planet Earth programme is one of the most shared, with thousands swarming around each episode.
The boss of peer-to-peer (P2P) file-sharing company Morpheus says it is the content providers who need to change their thinking.
And indeed some bridges are being built between content providers and the file-sharing systems.
A number of P2P companies have already befriended some music companies. They are supplying copyright-protected content for users to share, but it will only play after a fee is paid.
Certainly, in order to avoid going the same way as Grokster - and Napster before it - file-sharing companies do need to shed their association with illegal content.
The BBC is experimenting with a particular kind of file-sharing technique to allow subscribers to download recent TV programmes, as is satellite broadcaster Sky.
The open-source, free operating system Linux uses its loyal user-base to distribute operating system updates between themselves.
There are many legitimate uses for P2P file-sharing - but can the content providers be persuaded to use it, when the same application could be used to rip their material off?
The Internet Service Provider NTL is trialling a movie distribution service which uses BitTorrent.
NTL's Kevin Baughan says: "What we have done is put together P2P clients with storage facilities inside the network as well.
"So our goal is to create a superior performance for commercial legitimate content to be distributed.
"If we can combine that with very high speed access, which we're also trialling, we hope to really transform the download experience, and therefore be able to put commercial content in a different category to the current situation."
NTL is actually planning to insert copyright-protected material into a P2P network, which will then be shared by other users.
Mr Baughan says: "We're in negotiations with a couple of content providers, so together with them we'll take legitimate content, that we're allowed to use in the trial, and we'll place that and then see if we can actually create this superior performance.
"We already have an interesting hybrid. The client is free to go and find files wherever it wishes on the internet, and when it comes to choosing one that's from a commercial, legitimate source, then it changes gear, and it brings in this additional power from the network."
They are effectively prioritising good quality, legal content, but without creating a complete walled garden, says Mr Baughan.
"At the end of the day we have to think carefully about this with our own content providers, about what the right product solution is."
Technology analyst Jonathan Arber, from Ovum, believes this is the right kind of thing for ISPs to be doing.
He told Click: "I think for larger ISPs, such as NTL, the key way to really monetise P2P, to turn it into a revenue-generating application, and to simultaneously lower the amount of illegal file-sharing, is to offer a legitimate alternative, and in order to do that you've got to have something to attract users.
"I think increasing the speed of downloads is probably the key way to do that, as well as making sure there's a very strong range of content available, and of course pricing, but that comes in later down the line I think."
Rightly or wrongly, it feels as though P2P file-sharing has become a dirty word.
It may be difficult to persuade the big content providers and Hollywood studios that P2P can be cleaned up.
Kevin Baughan says: "I think there's several things going on at once here. Many of them are looking at the technology themselves, because there's a fundamental challenge. How do you actually distribute content?
"Video in particular is a huge amount of traffic, so the goal of the trial is to find ways to actually make that feasible.
"Now at the same time, that doesn't take away the need to police and make sure the right things are done on any illegal activities, but at the same time by creating a superior experience, then we really provide some carrots.
"I think the vast majority of people are keen to come and take legitimate content and consume it in a way that makes sense to everybody."
When it comes to responsibility for policing shared copyrighted material, Jonathan Arber says this is a very tricky area but ultimately it is not the ISP's job to enforce copyright laws.
Even if they are becoming content providers, he says it still would not be their responsibility to enforce copyright law.
"They would merely become another content provider, just as we do not expect HMV or Virgin to enforce copyright law.
"There are people who argue that perhaps ISPs should provide more information about what their users are doing, but that's a very thorny area to get into."