OmniHelp (OH) is a free Open Source LGPL cross-platform, cross-browser tri-pane Help viewer. It's the only one we know of that is not locked by licensing to a proprietary help-creation tool. Anyone can implement it with our blessings and support. The project is hosted on SourceForge.
OmniHelp is built for speed. When users need help, they don't want to wait around. They don't care how pretty the docs are, they care about finding the info they need. Now. So we don't sacrifice speed for fashion, or for the new tech that is usually slower than the old tech. We use the tri-pane that everyone knows, rather than some fancy frameless (means slower) format nobody has seen before. Loading is very fast, even from the Web, because we only load what is needed for the current request. Search is instant, because it is pre-indexed; it also provides progressive Boolean operations to widen or narrow the search scope, an exclusive OmniHelp feature, as well as regex search to select from the pre-indexed terms. If you create an index too, which we strongly recommend, you see exactly where each reference will take you before you click it; no flying blind. And both the index and contents expand and contract so you see just what you are interested in. Quickly.
For a real-world example, see our 750-page User's Guide in OmniHelp format. You can also download it for local viewing; OH works the same either way. We produced it using DITA source from over 1600 maps and topics using DITA2Go in just over 7 minutes on an older (2.3 GHz, 4 GB) Windows 7 system.
In addition, we have converted the DITA 1.2 Specification to OmniHelp. You can also download a zip of it (7.8 MB) for local use. That took DITA2Go a good deal longer, 70 minutes, due to the 300,000-plus xrefs it contains, all of which were verified.