Let's start a list of what ebook formats the iPad/iPhone can read, and what apps you need to get to read them!
Maybe something like:
DRM Formats, Vendor Specific:
iBooks (Fairplay ePub)
iBooks
Kindle (various formats)
Kindle App, by Amazon (iPad and iPhone apps available, no sideloading)
Barnes and Noble (Adobe Ignoble PDF and ePub)
Barnes and Noble eReader Apps, by Barnes and Noble (iPad and iPhone apps available, no sideloading)
DRM Formats, General:
Adobe Inept (PDF and ePub)
Kobo, by Kobo (iPad and iPhone apps available, no sideloading)
Txtr, by Txtr (iPhone app available)
Non-DRM Formats:
ePub
iBooks, by Apple (iPad app available now, iPhone app 6/21)
Stanza App, by Lexcycle (iPad and iPhone apps available)
IBIS Reader, by Threepress (HTML5 based, no app required)
PDF
Stanza, by Lexcycle (iPad and iPhone apps available)
Cloudreaders by Cloudreaders
GoodReader by Good-i-Ware
iBooks, by Apple (for both iPad and iPhone on 6/21)
Mobi
Stanza, by Lexcycle (iPad and iPhone apps available)
PDB
iSilo by iSilo (iPad and iPhone apps available)
Stanza, by Lexcycle (iPad and iPhone apps available)
Other:
Comics (CBR, CBZ)
Stanza, by Lexcycle (iPad and iPhone apps available)
Cloudreaders by Cloudreaders
http://itunes.apple.com/us/app/comic...=8] Comic Zeal by bitolithic.
Unsupported:
Sony Reader by Sony.
edit: There is a FAQ
here as well , thanks, nikkie!