See also new vibration tutorial video
The (unfortunately only one) time that I took a introduction flight in a Cessna 172, the instructor let me take the left seat and we did a couple of traffic patterns. After all the flight simming at home, one of the things that struck me was the amount of vibration from engine and landing wheels (The landings I made were not that smooth). Your rear, hands and feet get a lot of vibration feedback, especially when going to lower RPM settings (maybe the engine was running a little rough, as the throttle lever was really shaking).
You can add these kind of vibrations to your sim by placing Tactile Transducers at various places in the sim setup. Below your rear is the obvious place, but your hands and feet are much more sensitive to vibration, therefore I also placed a small transducer on the throttle assembly and on the rudder pedals.
The transducers are driven by the audio output. If your flightsim and aircraft have good wave files, with frequencies going down to 5 10 Hz, you will get great sensations from your hands when touching throttle or elevator trim. Your feet are surprisingly sensitive, and you need very little transducer driving power to get good feeling of engine, spoiler wind vibration, and wheels touchdown.
Flight Unlimited III has excellent low frequency sound for the Mooney Bravo, Stemme motor glider and Lake Renegade water plane. The turbo engines of the Beechjet dont do much for vibrations, but the gear down vibrations and landing on runway bumps are fine. There are also lots of downloadable airplanes at AVsim.com that you can try. You can also modify existing wave files and add some low frequency waves at key areas. Keep in mind that 5Hz 60Hz is the range of interest.
You can buy tactile transducers, but you can easily make your own for very little money and good results. Tactile transducers can be made from old low frequency speakers. You need to partly remove the paper cone, mount a metal strip to the speaker frame, glue it to the remaining cone (that is connected to the voice coil), mount some weight at the center of the metal strip, and thats it.
In this way, the voice coil now drives a heavy weight (0.5kg or so) but no surrounding paper. You can connect the transducer to an small power audio amp (20W car radio IC amps are ideal) which is driven from your PC sound card. (you need to mix L and R channel together, and add some low pass filtering (around 100Hz roll-off.) The transducer will produce vibrations in the 10 60 Hz region, but hardly any sound.
The procedure is described in http://www.hometheaterhifi.com/volume_4_2/tactile.html , (a great DIY site), which shows how to make big transducers for home theater.
For flight simming, you dont need very large transducers or high driving power.
The one mounted under your seat is the biggest, as the seat cushion takes some of the energy away. I used an old 8" Woofer from the dump that had damaged paper cone, but intact voice coil and suspension spin.
The one for mounting on the throttle unit can be smaller, 6" or so. Due to space reasons, you may not want a round speaker. Oval speakers work fine as well, but they need to be the woofer (low frequency capability) kind. These are often used in bigger size TV sets. I used a 180x60mm TV speaker type. If you get some rattling from loose parts in the cockpit controls box, you can use some cloth or adhesive rubber strips to fix things. Even your monitor may start to rattle, as vibrations get transferred from cockpit to table. I placed the whole throttle unit on rubber strips, then I taped adhesive rubber strips around the cockpit case at all the areas where it touches the table or supports. That solved most of it. It is really cool to see your throttle lever shaking after engine startup.
For your feet, you can mount the whole rudder pedal assembly on rubbers, and put one 5" transducer on the base board. It works, but I got a lot of rattle sounds from the springs. Therefore, I mounted very small 1.5" multi media speakers with some added lead weight glued on the cone, then mounted directly on the pedals themselves. With only 3Watts / speaker you get plenty vibrations, and little rattle.