Besides the basic information you find at this web site there are several alternative information sources. Best to start with is the SAGA project homepage hosted by SourceForge. Most up to date information is given by the SAGA WiKi, newsletters, bug trackers, mailing lists and user forums. In the download section you find Vern Cimmery's SAGA User Guide Volume 1 and Volume 2. Volume 1 contains an introduction to the graphical user interface of SAGA, while Volume 2 provides 'How To' information on many SAGA modules, functions, and GIS applications. The SAGA User Guide is updated to SAGA version 2.0.5. Still available and worth a look is Victor Olaya's Gentle Introduction to SAGA GIS, which has a strong focus on methods but dates back to SAGA version 1.1.
For user interface functionality SAGA uses the cross platform GUI library wxWidgets, which enables an operating system independent software development. Platforms currently supported by SAGA are MS Windows (2000 and newer) as well as Linux and FreeBSD. MacOS should be possible, but no experiences have been reported yet. More informations are available in the SAGA WiKi.
SAGA is a Free Open Source Software (FOSS), which generally means that you have the freedom
Except for the SAGA Application Programming Interface (API) most SAGA source codes have been licenced under the GNU General Public Licence or GPL. The GPL requires derived works to be available under the same or a comparable licence, with other words derived works have to become Open Source as well. For a few reasons the SAGA creators decided to use a less restrictive licence for the SAGA API. The API uses the GNU Lesser General Public Licence or LGPL, which permits use of this library in proprietary programs, i.e. SAGA modules, which always base on the API, have not automatically to be published as Open Source too.
Under Windows this is very simple, because the Windows version of SAGA is a so called portable software. Just download the latest binaries compiled for Windows unzip it and run 'saga_gui.exe'. Under Linux there is a similar way to run SAGA. Get the binaries compiled for Linux and execute 'saga_gui.sh'. Due to unresolved dependencies in your Linux configuration this might not always work immediately. In this case look for more information on how to run SAGA in the SAGA WiKi.
Since SAGA is not a commercial product and all people involved in its development have to do other jobs in the first, there is nothing like a hot line or a guaranteed support for your special problem with SAGA. If you need help with regard to SAGA, the best is to check out the WiKi, forums and trackers first. Of coarse, if this does not help, you might also contact the SAGA developers, who will always give their best to find a solution for any problem, though it might take a while.
There are many ways to support SAGA. Direct contributions can be done through SAGA's project homepage at SourceForge, where you can participate in forum discussions or improve the WiKi. Become a funding member of the SAGA User Group Association, which represents the interests of the world wide SAGA user community. Those of you, who want to contribute as developer, should contact the development team which welcomes any enlargement of its capabilities.