Led Zeppelin will re-release their first three albums on June 3.  Guitarist/producer Jimmy Page has remastered each album.  The band plans on releasing remastered versions of all their albums eventually.

If you want the "deluxe editions" of these albums, you will get previously unreleased material on companion discs.

Page says, "The material on the companion discs presents a portal to the time of the recording of Led Zeppelin.  It is a selection of work in progress with rough mixes, backing tracks, alternate versions and new material recorded at the time."

With the band's debut album, Led Zeppelin, the companion disc will include a concert from October 10, 1969.  The concert featured seven songs from the band's debut and two songs that would be on the band's second album.
The companion disc for Led Zeppelin II will include alternate mixes for five songs from the album along with a previously unreleased song, La La, and backing tracks for Thank You and Living Loving Maid (She's Just a Woman).
The Led Zeppelin III companion disc will feature seven studio outtakes and previously unreleased tracks called Jennings Farm Blues, Bathroom Sound, and Keys to the Highway/Trouble in Mind.
See more at LedZeppelin.com.