Sublime Text for Windows, Mac, and Linux TextWrangler for Mac TextMate for Mac
Windows Notepad (should already be installed on Windows machines) Atom, a new editor for Windows, Mac, and Linux. Early days, but seems to be quite featureful. Free.
Windows Notepad (should already be installed on Windows machines) Notepad++ for Windows
vi/vim or emacs for Linux (should already be installed)
Kindle Previewer Amazon's free Kindle Previewer is available for Mac and Windows. It lets you convert your EPUB files to Kindle format and preview it in simulations of several different Kindle models. KindleGen KindleGen (also free from Amazon) is available for Linux as well as Mac and Windows. It is a command-line program. Calibre Calibre is a free ebook management tool that (among many other functions) can convert to and from most common ebook formats.
GIMP is a free image editor for Windows, Mac, and Linux. MorgueFile.com has a large collection of images that you can use as a base for your cover. Wikimedia Commons is another good source. Check the license to make sure it's okay to make commercial use of the image. Flickr has millions of photographs available. To find images that you can use in your cover, go to the advanced search and check "Only search within Creative Commons-licensed content", "Find content to use commercially", and "Find content to modify, adapt, or build upon" (other images on Flickr will require explicit permission from the artist).
There are also many stock photo image sites on the web where you can purchase image rights for a reasonable fee. Sarah Hoyt has written a good series of tutorials on basic cover image creation.