The only single from the album "Ghost Reveries" is "The Grand Conjuration". A music video of the song was released, although about half of the song was edited from the video, due to the length of the song. Drummer Martin Lopez does not appear in the video, as he was sick and was temporarily replaced by Gene Hoglan.
The song Ghost of Perdiction is featured in the soundtrack of the videogame Saints Row 2. It can be heard on the radiostation "Krunch 106.66"
Contrary to popular belief, Mikael Åkerfeldt isn't one the founding members of Opeth. He joined Opeth in the same year of its formation (1990) as a bass player.
Mikael Akerfeldt is the inspiration for the design of Toki Wartooth, rhythm guitarist in the cartoon band Dethklok.
The track "For Absent Friends" on the album "Deliverance" was named after a song of the same name, originally appearing on the album Nursery Cryme by progressive rock group Genesis.
The song "The Night and the Silent Water" is about the grandfather of Mikael Åkerfeldt who had died shortly before the recording of the album.
Opeth won a Swedish Grammy with Deliverance for "Best Hard Rock Performance".
The track "Master's Apprentices" was named after the seminal Australian hard/progressive rock group The Masters Apprentices.
Blackwater Park is titled after an obscure German heavy progressive rock band of the same name.
Mikael Åkerfeldt dedicated Deliverance and Damnation to his grandmother, who died in a car accident during the recording.
Mikael Åkerfeldt & Peter Lindgren were given #42 place in Guitar World's "Top 100 Best Heavy Metal Guitarists of all Time" (March, 2004)
Opeth have taken inspiration from 70s British prog/folk rock band Comus. The album name 'My Arms Your Hearse' is derived from lyrics from Comus' song "Drip Drip," and the song name "The Baying of the Hounds" is taken from the song "Diana." Both songs appear on Comus' album 'First Utterance.'

Wikipedia entry on Comus
The name Opeth was taken from the book Sunbird (1972) by author Wilbur Smith, and was originally spelled Opet. In the book, Opet is the city of the Moon. Mikael Åkerfeldt claims that the book is "shit".
In the album 'My Arms, Your Hearse' each song's lyrics end with the name of the next song, and the last song, Epilogue, ends with the name of the first song, Prologue.