MadCap Flare ($1000, 30-day free trial) occupies rarified territory near the top of the desktop publishing mountain. It is intended primarily for those who need to author, edit, and produce very long and very complex documents: technical manuals, instruction books, help systems, and anything else in which structuring and organizing content is key.
While a typical word processing program is often focused on ease of use, and a desktop publishing program on design of output, Flare's strengths are in content management and is driven by a "write once, publish anywhere" paradigm. Documents in Flare are composed of Topics, which can be of any size. Topics are organized in the Content Explorer, using a typical hierarchical folder system. The output of the topics is determined not by the Content Explorer, though, but by Tables of Contents--of which you can have several for a single project, depending on your output needs. Thus, you can group all Topics on a single subject (or perhaps all those written by a single author, as Flare supports collaborative work) in one folder or tree in the Content Explorer, and then output them in many different places in the final document. (As an example, you might be writing a guide to an API. In the Content Explorer, you might want to group all "String Functions" into a single folder. In the published document, though, Chapter 3 might contain the string-manipulation functions and Chapter 5 might contain string-conversion functions.) A single topic can appear in multiple places in a document.
Further, Flare allows for, and expects, publication in multiple formats. Flare encourages users to use styles ("Header 1," "Body Text," "Example Code") instead of specifying the font or color of text directly--though you can do that if you need to. When publishing, text in one style ("Header 1", say) can be given one set of characteristics for PDF publishing and one for HTML. You can have multiple outputs in the same format as well--you might want to use different fonts for PDFs intended for different markets.
Furthermore, Flare supports conditional text--text which is output in some formats but not in others. This can be used in many ways--a textbook that has a "Teacher's Copy" with answers or notes, or a report intended for several divisions in a company that has sections of interest to one division but not the others.
It's also good to discuss what Flare doesn't do. While it does allow a lot of control over the look of the output, it's not intended for documents with complex and artistic graphical design with pixel-by-pixel control over each page. Flare can be used to create content directly--and if you're mostly dealing with hundreds or even thousands of small topics, this might work well--but it's not a word processor and shouldn't be used in place of one. Because of how it handles content, it helps to have a good idea where you're going before you start.
I also encountered some bugs. At one point, I had some issues with file paths getting out of sync, and this caused some serious problems. Another bug, which was very frustrating (and which has been acknowledged by MadCap) is that detaching windows from the main frame caused the text editing toolbar to fail to activate.
If you're in an environment where the production of complex documents is common, and especially if you often need to produce output that looks good in many formats (and don't want to hand-edit it time after time), MadCap Flare is worth looking at. It is by no means cheap, but its price is comparable to that of the leading tool in its category, Adobe FrameMaker. The thirty-day trial version gives you almost all the functionality, but the output text will be garbled with random characters. This is more than enough to judge if it can meet your needs, however, and the support staff is very responsive.