Accufeel & 3D Real Cockpit. These are pretty inexpensive enhancements to any airplane, if you haven't given them a try. If you've seen cockpit videos, you notice that they don't seem as velvety smooth as they are portrayed in flight sim. Certainly in the venerable Cessna trainer I used to take lessons in, landings could be something of an adventure when I was in control.

Accufeel is from A2A and creates a nice stereo soundscape linked to what's going on with your airplane. Taxis get a bit of rumble and bump, there are brake squeals, and groans. Takeoffs and landings give you a nice rattle. There are tire squeals, too, based on your landing. You can modify effects with a slider. For instance, your rattle can range from loose bucket of bolts perhaps suitable for one of your beat-up old 727s, to tight and brand-new. The sounds tend to be on the loud side, but that's adjustable, too. Maybe once the novelty wears off, it won't be as much fun, but I'm enjoying it now.

There is also 3D Real Cockpit who gives you shudders and shakes depending on your taxing technique, and your airplane. Take off can set your airplane shaking to the point you hope it gets in the air before it falls apart! It, too, is scalable. That's a lot of fun.

I've been using them together, and they really add an element of immersion.

Oh, another one is Cabin Crew. Again, works with any airplane, and comes in different languages. All of these sort of products add to your experience, much the way GSX does at the gate.

Finally, I've been trying out FSX booster. This is one that I'm sure many will condemn as snake oil, and say you should learn everything about your system and tweak it to death, but this is interesting, at least. I can say it has killed the blurries. Since it encourages you to limit your frames, it's hard to quantify results. If nothing else, it educated me about the relationship among various factors. True, no dramatic 300% increase in FPS, but my sim is solid and smooth.

None of these products are very expensive. Probably the most instantly gratifying is the 3D Real Cockpit. It's nice to have Spanish cabin announcements on those South American flights, too! I'd be interested in hearing other users' opinions.

Flying from KMIA to KATL Delta 757 (actual equipment). Three greens, my friends.

You can adjust the effects to whatever you think is realistic or fun. I have always liked the sound of a rattling landing that I have heard on the rare flight sim airplane. This way, you can have all the neat sounds on all your airplanes. I was taxiing a 757 and it was groaning on the turns, and the breaks were squealing and there was a rumble and my wife looked up from her knitting and said, "I don't think I would want to ride on that airplane." So, yeah, you can over-do it nor not. I think to does enhance things. Watch some in-flight records of landings and take-offs and decide for yourself if the completely smooth atmosphere FSX gives us is realistic.

The 3D Cockpit is entirely different and more "in-your-face." Literally. It does a nice job of shaking your airplane. It, too, is adjustable. By shaking, that's just what I mean: shaking the VC. You can overboard with this one, too. I had it maxed out just for fun and extended the airbrake. You would have thought it was going to shake itself apart! Obviously, you probably don't want that. But you can dial it way down, too.

I would say if you're looking to spice up your flights even on a big airplane (that's all I fly) one or both is worthwhile. Remember, AccuFeel translates physics into SOUND; the squeal of tires on landing, the "scrub" of tires turning on the taxiway, wind, etc. The other is a VISUAL effect that shakes your airplane. I think both of them are good. The 3D Cockpit effect is definitely more dramatic. 

Some movement, yes, but not like the shake-your-fillings-loose vibration and shaking you can get with the other product. I'd say a different kind of movement. Like with AccuFeel I've noticed turbulence out of nowhere that will push you a bit, and you can hear it over the wind. Again, both totally customizable.

EDIT: Let me sharpen the differences a bit after some testing. When taxiing with ACCUFEEL, you have to be careful because you really get the sense that you're driving something with enormous, heavy wings filled with fuel on either side, not a car. It is not hard to sway a bit, and you can even scrap a wingtip! (I don't know how common that is in real life, but at least it makes taxiing more interesting.) There is also occasional clear air turbulence, which pushes you around a bit.

With COCKPIT 3D EFFECTS you do NOT get that sort of movement. Instead, it is chattering, shaking, jittering. More of a visual effect than a movement effect.