Slipknot and Madness are the latest names added to the Roskilde bill.
Today's announcements join Coldplay, who are the only other act confirmed for Denmark's biggest festival.
Oasis are also expected to play at the event, as exclusively announced by Virtual Festivals, but nothing has been confirmed yet.
Roskilde Festival 2009 will take place at Roskilde, Denmark from 2-5 July.
Tickets are on sale now priced at £190 (plus booking fee).
The festival shares a similar ethos to Glastonbury, placing equal emphasis on the peripheral goings on as its big name acts.
The festival features various workshops, sports and communal activities and is run by volunteers, with a strong green message.