What is SynthFont[1]?
SynthFont[1] is a free tool for playing MIDI files making use of SoundFonts ("SF2" files) or other sound files in formats like DLS, GIGA, 404, GUS. SynthFont[1] is the original version of SynthFont and has now a younger sibling, SynthFont2, which is not freeware. Read more about SynthFont2 here, Much what is said here about SynthFont[1] here is also valid for SynthFont2, and will not be repeated in the tutorial for SynthFont2, so it is a good idea to start here.

With SynthFont (1 or 2) you do not need any particular sound card. In fact, if you only want to render audio files (".WAV" files) you don't even need a sound card.

One major feature of SynthFont is that it very easy to assign different SoundFonts to difference MIDI channels and also any SoundFont Preset of your choice to a particular MIDI program in a MIDI track, like in this image (SynthFont2). These settings, and many more, can be stored in an Arrangement file. (Note the buttons in the toolbar above the tracks list!)

The first thing to do before loading a midi file is to select the default SoundFont. SynthFont will ask you to do it the first time you run it. You can change the SoundFont later on (go to the File... menu to find this function). The default SoundFont is used whenever an "unknown" midi file is loaded for which there is no SoundFont information available. (Note that if there exists a SoundFont file having the same base name as the midi file in the path, then that file will be used and assigned to all channels instead of the default SoundFont.) The default SoundFont should preferably be a GM (General Midi) SoundFont, i.e. a SoundFont with programs/presets for all 128 instruments defined by the GM standard. The SynthFont installation package includes the most basic GM SoundFont of all: the SYNTHGMS (or the larger GMGSx) . If you want a better alternative then go here and download one or two.

1 Files / Folders - to use the file Playlist or the inbuilt Explorer
2Pianoroll - to inspect and edit the midi file
3 Midi events - for in-depth inspection and editing, only for die-hard experts!
4 Plug & Play - this is where all the fun begins. The image above shows this screen. At the top you have the "synth rack", with a few things you can change. Below this you have the tracks list, which also gives you access to a number of parameters. For example, to change the SoundFont for a particular track, select it and press the "SF File" button in the list heading or then "SoundFont File..." button in the main toolbar.
5 Preset layers - only of interest if you want to see what a SoundFont contains
6 Splits in layer - only of interest if you want to see what a SoundFont contains
7 Sample data  - only of interest if you want to see what a SoundFont contains
8 Info - some information may be displayed here

The tabs 1 and 2 are visible in Just Playing mode, while tabs 1-4 are visible in Midi Guru mode.
Below is the main window depicted, in Expert Wizard mode. There are three user levels: Just Playing, Midi Guru and Expert Wizard. You can change mode in main menu View.
In Expert Wizard mode you have access to eight tabs:
More about Plug & Play
In SynthFont2 the selection is slightly different:

SynthFont[1] tutorial, part 1