Ok, I think it's time to start to write some documentation.
The manual should document all options in the menus (the options in the preferences dialog will have help texts, so it's not necessary).
I'll write it here, in no particular order. In this moment the important is the content and not the aspect, so there'll be no bolds, italics and so on.
If someome want to help me, just edit the page and write the documentation for the options you like (or fix or improve what I wrote). I'll review later and add some detail if necessary.
Open->VCD. It will try to play the disc as a VCD. All titles in the disc will be added to the playlist. It will use the CD device configured in Preferences->Drives. If you try to play a VCD but the CD device is not configured yet, then a warning will appear and you'll have the chance to configure the CD device. After that you'll have to select this option again to actually play the VCD. Note: this option will work on Windows too but it requires a recent version of mplayer (from November, 2007 or so).
Open->Audio CD. It will try to play the disc as an audio CD. All titles in the disc will be added to the playlist. It will use the CD device configured in Preferences->Drives. If you try to play an audio CD but the CD device is not configured yet, then a warning will appear and you'll have the chance to configure the CD device. After that you'll have to select this option again to actually play the disc. Note: it seems there are no mplayer builds with support for audio CDs so this option is disabled under Windows. If you want to try anyway, you can enable this option by editing the smplayer config file (C:\Documents and Settings\your_username\.smplayer\smplayer.ini) and setting to true the option enable_audiocd_on_windows.
Open->Close. Closes (hides) the smplayer window and stops the current file. If the option "Show icon in system tray" is enabled then smplayer will keep running hidden. If that option is not enabled then smplayer will exit.
Open->Quit. Quits the application, no matter the state of the "Show icon in system tray" option.
Play->Stop. Stops the current file. If you want to reset the counter press stop again.
Video->Compact mode. Hides the menubar, statusbar, toolbars... Press Ctrl-C to exit this mode.
Video->Filters->Postprocessing. Activates a postprocessing filter which can make the video look better by trying to hide some of the defects of the video image, like blockiness.
Video->Filters->Add black borders. Adds a black border around the image according to the aspect ratio of your monitor (if you have a extended desktop, it will use the aspect ratio of the screen where smplayer is). The black border can be used by the subtitles.
Video->Filter->Software scaling. Upscales the video image to fit your screen. This process is slow, so you need a fast CPU.
Options->View logs->MPlayer. A window will display the output messages from mplayer. This can be very helpful when there's trouble. For instance if a file doesn't start to play, you'll probably find the reason in the mplayer log.
Options->View logs->SMPlayer. This displays the debugging messages from smplayer. It includes the functions that are called and maybe what they're doing.
SMplayer Preferences Explanation:
If checked, turns on direct rendering (not supported by all codecs and video outputs)
WARNING: May cause OSD/SUB corruption!
To improve DVD playback navigation (i.e. FF/REV, time-skip forward/Backward), turn on DVD Caching and set to 10Mb (i.e. 10000 k) in the following menu:
Options->Preferences->General->Media Settings->Remember time position
If checked, your last time position within a file will be remembered. When you start playing that file again, it will begin from the same place. For example, you close SMPlayer an hour into a movie. If you open that same movie again, it will resume playing from time position 1h00m. Beware - this setting will *not* work correctly if the 'Play files from start' option is checked in Playlist Preferences. This option can be found by clicking the 'screwdriver' icon in the Playlist window.
Options->Preferences->Performance->Cache increase DVD cache size to 10000 and your ability to Fastforward or Rewind DVD playback will improve by an order of magnitude; especially for DVD drives installed in most laptops that provide minimal HW buffering.