The e-publisher's basic toolkit

HTML/CSS/JS editor

TextWrangler: barebones.com/products/textwrangler
(or barebones.com/products/bbedit $)

Format conversion

Calibre: calibre-ebook.com

EPUB editor

Sigil: code.google.com/p/sigil

EPUB validator

FlightCrew: code.google.com/p/flightcrew
(alt. threepress.org/document/epub-validate)

ebook viewers (for Mac/PC)

NOOKstudy: barnesandnoble.com/nookstudy
Kindle for Mac: amazon.com/gp/kindle/mac (also for PC)

Cover image creation

tjshome.com/imageconverter.php
tuxpi.com
resizr.com
pixlr.com
cosketch.com

Printing ebooks

epub-to-pdf.com
pdfonline.com/pdf-to-word-converter

Note: I have used all of these (the online versions, if available) on MacBook Pro (OS X 10.6.8)