The original book, 8888, is probably the best of them. The last two models in it are particularly interesting, and it took me a minute to figure out what they do from the pictures. They use a very clever idea of accepting code cards made up of rows of 1x4 gear plates, which allow you to program the models in a purely mechanical way. There is also a wind-up clock and an excavator with 3 degrees of freedom. Technic (or Expert Builder, rather) was really in its infancy back then, and it's quite impressive how much they managed to achieve with the very limited selection of Technic pieces available at the time.
8891 also has some good concepts. There is a plane "simulator" (similar to the 8485 helicopter), a walker with pneumatic arms and hands, and a rodeo guy on a bull. I especially like the pictures for that last one.
(update: pictures and comments appear below)
A simple but very fun model. One motor makes the bull jump up and down, and the other one makes it rotate. If you use both functions at once, the speed of the jumping changes noticeably depending on the direction the turntable is moving in, so if the guy won't fall off, you can speed it up.
Great concept. I often tried to build things like this as a kid. It moves fairly slowly and does wobble a bit while walking, but still remains surprisingly stable even with unbalanced arm positions. I changed a few pieces on the hands to make their grip stronger. As you would expect, it benefits a lot from a compressor and there is probably enough unused room inside to build one into the model itself, although I just ran it off an external one. The cockpit can be opened to put the figure in. Here is a walking video.
I love the concept behind this model, but its operation is kind of unreliable in practice. I removed the crate that the model normally lifts, which is too heavy and doesn't let the winch stay up. The turntable mechanism is unusual and tends to lock up, and the model would definitely benefit from a modern turntable. It's impressive to watch otherwise though. Each of the six rows on the "code card" corresponds to a function: turning the crane left and right, raising and lowering the winch, moving the tracks (they can only move forward), and moving the card itself. As Blakbird mentioned, the model has heavy gear reduction and it takes around 3 minutes for the card to fully run through the model. I think this model might also hold a record for the number of 8-tooth gears used. I counted 24.
This one is probably my favorite among the models I've built so far. It's pretty small and compact, but it has 3 degrees of freedom like a real excavator. It's interesting that TLG managed to achieve this in 1980 with very basic Technic parts, but couldn't do it recently with 8294. The original model uses a rubber band in one spot to keep the boom up, which doesn't work reliably and causes friction in one of the geartrains, so I removed that and added in manual latches on the cranks as seen in the first picture.
This is a great idea with a somewhat flawed design, but it can be fixed easily. The motors aren't geared down enough and some rubber belts in the geartrain tend to slip easily, so the plane moves with fast, jerky movements and can't hold its position well. I removed the belts and added an extra 1:3 stage of gear reduction on each motor, which improves the functions a lot. I also enlarged and motorized the propellor and added a few supporting plates on the wings, which are otherwise prone to falling off. After these changes, I think I like this model better than the 8485 helicopter, as it has similar functionality in a much smaller size. I took a short video of it as well.
Another walking model, this time with 4 legs. It moves fairly fast but not quite as smoothly as the 2-legged walker discussed earlier, and it actually moves better backwards than forwards. The legs tend to slide on a hard surface, so I added some wheels on them for traction. They look a little weird but they do the job.
Like the crane, this is another very unique concept that unfortunately doesn't work that well in practice. The pendulum has two bushes on top that touch a 40t gear. The gear moves by one tooth each time the pendulum swings from one side to the other, which makes the clock hand move just a little bit. The entire thing is powered by several rubber bands on a wind-up axle, and the 24t gear in the back is used to wind it up. The gearing is quite clever and is designed so that it can be wound up easily, but still keeps going on a single "charge" for a while. However, the pendulum does not swing reliably, and also has a slight deadzone that causes the gear to slip every now and then. I added a few weight bricks to it, which improves it but it's still not that reliable. There is also a cut-out clock face given in the book, for which I just used a substitute.
Another cool and unique model. The cable car has a forward/reverse transmission that gets flipped when it hits a wall, automatically reversing its direction. The string it runs on needs to have quite a bit of tension in it, or the wheel doesn't grip the cable tightly enough and the car will not move. It took me some time to figure this out, but the model is fun to watch in action once it has been set up correctly.
This model uses a symmetric mechanism for the front and hind legs, unlike the dinosaur. It actually walks very slowly, but is reasonably stable. The head bobs up and down as it walks forward, and it also has a bone in its mouth. Here is a walking video.
Edited by CP5670, 31 August 2010 - 10:25 PM.