While the content (the words themselves) are most important, the way we put them down can affect how productive we are and how our words are read by others. Over the years, I've developed an assessment of which means work for which ends, and I thought I'd share some of these company secrets with those of you who may be looking for alternatives.
For me, the simplest and often the best is plain text. It transports well in email, and it serves admirably in many other situations. It's easily searched (think "grep") for ideas later. It's easily signed or encrypted. Files are small. The most productive way I've found to create text is GNU Emacs. Period (or full stop). With a few add-ins such as its table functionality (table.el), it's easy to create nicely formatted, lightweight documents people can make sense of easily. With a few more add-ins, you can do almost anything you want: read mail, create Web sites, make calculations, store data, and plot graphs.
Sometimes plain text won't do. It really is hard to do better than LaTeX for producing nice looking documents to be printed or to be viewed as a PDF file. I've found AUCTeX a great add-in to Emacs to make it easy to write LaTeX.
In an increasing number of cases, it's important to be able to use the same text for multiple applications: print and the Web (and even online help), customer A and customer B, etc. For books, articles, and especially for documentation for computer hardware and software, DocBook provides a good solution. Write the text once, and then process it differently for each. It makes filling in the appropriate customer name or including the appropriate sections for each customer (relatively) easy. Emacs, with its nxml-mode, is again my tool of choice for creating DocBook documents. Perhaps the biggest challenge is selecting a "toolchain" that produces the documents you want: producing HTML is almost trivial, while producing PDF can be harder. DocBook can even be suitable for really quick, short documents.
Maybe you want a GUI, so you don't have to remember as much about how to format things the way you want. I've found OpenOffice.org's Write to be handy. Most importantly, its styles features make it easy to create structured text that looks good and is easy to maintain. Starting with version 2.0 (coming soon, they say), OpenOffice.org will store documents in OASIS OpenDocument XML format, a vendor- and implementation-independent format.
These are simply my impressions, so "your mileage may vary." For more information about any of these approaches, see these links.