Soft Synths are a handy way to save £££s (or $$$s or ¥¥¥s). They also improve the sound of MIDI files if you've got a cheap'n'nasty sound card, but slow down your games or other currently running applications.
There are basically two types of Soft Synth.
The first are software versions of General MIDI (GM) synthesisers, like Roland's Virtual Sound Canvas. Yamaha have also produced a soft synth which uses XG (the Yamaha variant of GM). You need a Pentium to run it. It is one of those 30-day trials that you can use for 90 days. A window pops up when you load the program. Be careful though, I have heard that this piece of software thinks it owns the computer and is not easy to get rid of. However, I have not tried it so I can't say whether this is true. DirectX also has a GM set. However there seems to be no way of playing midi information to it directly.
Soft synths like the VSC-80 take up CPU power & RAM space, so the better your computer's spec, the better the performance you'll achieve.
The second type of Soft Synth is used essentially for the creation of sounds, although some have real time playback capabilities. They take various forms and mimic various synthesis techniques.
In addition to the many commercial softsynths available there are a growing number of freeware/shareware softsynths around. These include Subsynth (very simple subtractive), Granulab (granular), Defractor, Simsynth, SMorph, Orangulator (10 oscillator additive, subtractive, etc.), SynthEdit, FX2, and many others. Some, like the amazing Buzz are integrated sound and effect generators and Trackers. There are also a growing number of free VST instruments to be found on the net. A look around a site like Synthzone will point you towards these instruments, and many others.
My favourite softsynth is SynC Modular. You can build your own synth and effect structures using either the library of prefabricated modules or the basic functions. SynC Modular can be used as a Cubase instrument. You can design your own panel layout/appearance. You can also define your own "skins".
Many of the Softsynths mentioned above are beta or freeware. They are therefore not all as stable as you might like. However I've found all of them useful, despite the occasional crash.
At this site you will find the following Softsynth resources: