If you have a modern computer with some decent CPU power and memory, I like ZynAddSubFX for a standalone sythesizer. It has a beautiful graphical interface, a built-in virtual keyboard and effects, and is not too difficult on which to navigate and manipulate sound. It is user-friendly for a synth newbie but full of sound variations.
If you do not want quite the bloated package, AmSynth is attractive and straightforward in appearance but quite easy to manipulate for a standalone synth. It comes in a very small package so it is perfect for the minimalist computer and the new synth user. On the other hand, AmSynth does not have quite the sound flexibility and functionality of ZynAddSubFX.
Another powerful synth in a small download package is MX44. This little program could keep you busy for years just playing with all of its adjustments and nuances. The biggest drawback is the graphical interface. The GUI is huge in size and takes up the whole computer screen and cannot be resized. It has sliders and click-boxes going this direction and that with no labels or markers as to what performs what adjustment or function. You can figure it out with a little work but learning how to use it is rather painful for the musician who wants to make MusicProgramsmusic now rather than later.
Next up are the standalone DSSI-based packages: Hexter, Whysynth and Xsynth. They have similar graphical interfaces which are a little dated and not quite as easy to navigate and manipulate for a newbie. Whysynth and Xsynth have some crossover in terms of similar sounds, whereas Hexter is an attempt to copy an old Yamaha DX-7. These are all fairly light on the CPU usage and package size and even allow you to test notes without a virtual or real keyboard for your computer. Once you get used to graphical interfaces, you could use all three of these without any problems.
Once you start delving into synthesizers, you will start to get interested in modular synthesis. RTSynth is an interesting synth because it looks like it is trying to be modular but not quite. The idea with RTSynth is to connect adjustable synth instruments and effects together like modules, just like a guitarist connects a guitar to different effects pedals and amps. Like the DSSI-based synths, RTSynth comes in a small package but is a bit better-looking than the DSSI synths. RTSynth is probably a good entry into putting sounds together with modules, which leads you into the modular synths...
Spiral Synth Modular and Alsa Modular Synth are big, powerful modular soft-synths. If you lived to be one hundred years old you would probably not exhaust all the options offered by these two programs. Spiral Synth Modular is a bit older in appearance but is also 1/3 the download size of Alsa Modular Synth. Spiral Synth Modular also has a neat looping effect called Spiral Loops which I really need to learn more about but apparently has some potential for interesting effects. Alsa Modular Synth will give you some roadblocks if you try to add certain modules without appropriate LADSPA plug-ins, but I have included lots of LADSPA effects in the dotpup I provided so as to avoid this problem. Caution: Neither Spiral Synth Modular nor Alsa Modular Synth are easy to use if you do not have some synth and/or modular synth background. You will really need a modern computer with decent CPU power and memory in order to build anything interesting with these two programs.
Lastly, I will mention the other programs that do not fit well into the usual soft-synth category: Bristol, Fluidsynth and DSSI-VST. Bristol is a copycat standalone soft-synth, designed to reproduce and emulate a number of old synth and keyboard sounds (Rhodes, Juno, Prophet, Moog, etc.) right down to the GUI for each instrument which attempts to look like the actual keyboard. Bristol does a fairly good job at some of the sounds and the keyboard pictures look nice, but the graphical interface is clumsy to use as are the virtual keyboards attached to each instrument. DSSI-VST is essentially a shell for running Windows VSTi instruments. Find yourself a Windows soft-synth you like and try to run it in Linux with DSSI-VST. Some Windows VSTi instruments work and others do not, so it is hit or miss with this program. Fluidsynth is not really a soft-synth but a sample player with which you can add effects to your samples. You can save a lot of CPU usage if you simply load up a Soundfont file with Fluidsynth, turn off the effects and play to your hearts content. Of course, that kind of drifts away from the whole concept behind variable sound synthesis.
Overall, each program is capable of unique sounds and each has its use in music-making. Personally, I think ZynAddSubFX is a good complete-package synth for newbies and experienced synth users with a modern computer. For old or low-end computer users, I like the DSSI-based synths, particularly Whysynth, and MX44. To me, Alsa Modular Synth is really the king of the mountain when it comes to the synths I have compiled, but I do not yet have the adequate computer setup or the expertise to get the most out of it. Fluidsynth is nice if you want a sampler instead of a true synth, and DSSI-VST is useful if you are transitioning from the Windows music world to Linux. Good luck and play on!
