A music site that has captured many estranged Napster users and now claims to be more popular than its high-profile predecessor, caused a stir among music swappers on Sunday by halting its service and forcing them to download a new advertising-based client. Reports of a declining trade in MP3 files following the legal suit that has forced Napster to try to stop its users sharing copyright protected files have ignored the migration of users to other services based on Napster's technology. These services, running technology cloned from Napster's centralised servers, known as OpenNap servers, have blossomed in recent months. One of the most popular, belonging to MusicCity.com and functional up until Sunday, typically offered in excess of 20TB of data. At their peak in popularity Napster's official servers offered around 12TB. On Sunday evening, however, all trading through MusicCity's servers was halted and all users were sent a pop-up message telling them to download a new client, called Morpheus. MusicCity also appears to have attracted a significant number of ex-Napster users to this new client, claiming to already have over 27,000 users on its first day. The new client is similar to Napster and other services such as Gnutella, in that it searches and lets files flow directly between users, a technique referred to as peer-to-peer. Like Gnutella, Morpheus is designed to enable users to share all kinds of media, not just MP3 files. While operating an OPenNap server, MusicCity was always committed to being free. It remains unclear, however, how MusicCity intends to make its new technology pay for itself, whether it plans to charge users, and how it may try to fend off attention from the Recording Industry Association of America (RIAA), which has caused Napster so much strife. The company has previously committed to keeping the Morpheus client free. A spokesman for MusicCity, who describes the company as a startup, refused to be drawn on any of these points when contacted by ZDNet. He said that the firm's position would be made clear in coming weeks, with the launch of another Morpheus client. He also said that the fate of MusicCity's OpenNap servers was currently "up in the air". He did not rule out the possibility that source code behind the technology might be published, allowing a wider community of programmers to contribute to development of its usability and functionality. Morpheus currently runs on Windows only, and while there is a promise of a Linux version at some future date, there is no mention of versions for the Mac and other operating systems. The Internet is changing the face of the music industry forever. Find out more at the MP3 Newsroom. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Napster Debate. Let the editors know what you think in the Mailroom. And read other letters.