There are a number of gadgets that would make any touring trip more pleasant and efficient, while weighing very little. An incidental benefit is that in addition to being impressed by your bicycle, the local people are impressed by these gadgets as well.
A GPS (Global Positioning System) can save you hours in third world countries. Many times (before GPS) I took wrong turns costing hours since road signs are usually nonexistent on roads other than main highways. The GPS gives your latitude, longitude, altitude, points you North, and draws a map of where you've been. If an intersection you reach is not on the maps you carry (roughly about half the time), the GPS gives good information that increases your chance of making the correct decision. I have a Garmin GPS 2CX which cost $250. It has the longest rated battery life. Detailed information on how it works is available is at http://www.garmin.com/aboutGPS/. You should also bring a miniature compass, like the type that attaches to your watchband.
It's probably unwise to bring a GPS to Russia as someone was charged with espionage just for using a GPS. He was released after about a month. I recommend that in most third world countries, it only be used when no native is seeing you. In fact, if there was a native there, you probably wouldn't need to use the GPS because you can ask for directions. (Americans who don't travel are shocked that you can ask directions in any country by saying the name of the town, lake, etc. and the natives simply point the way.)
An altimeter gives more relevant information than an odometer in mountainous terrain. Energy expended (or still required) is more a function of altitude gain than distance. It also give you accumulated altitude gain for the trip if part of the cyclocomputer. Altimeter watches are fine, too, but don't get a watch-compass. Use the $100 increased cost toward a GPS instead (or get a simple $6 watchband compass from REI).
A wonderful gadget is a short-wave radio, especially for evenings if there's not much to do. I received Australia, Japan, China, India, Arabia (some Arabic music is quite erotic), many European countries including the BBC from London, and the Americas (including a few native American languages) from Costa Rica. Most major countries alternate broadcasting in English and other languages. The radio weighs only 8 oz and is 4x3x1" . This amazing size also contains a speaker and has AM and FM. (I bought my Sony SW20 in 1991. Grundig now makes a 7 oz. model). Even in areas that had no AM or FM, I still always received world-wide short-wave. After sunset, reception dramatically improves by about 30 times more stations than daytime. It uses two AA batteries which lasted over 60 hours.
Recent advances in optics have yielded much smaller binoculars and cameras. Binoculars are a must for observing wildlife. I saw three species of monkeys and many tropical birds (including the red-blue-yellow great macaw). There are several manufacturers that make binoculars that weigh under 10 oz. and are 4x4x2" (e.g., Nikon 10x25 Venturer II from 47th St. Photo 800/221-7774, $80.)
I stopped carrying an SLR camera years ago. I've had great reliability with the Yashica T4 and it's weatherproof. It is 35mm fixed focal length, but now many manufacturers have light zoom-lens cameras. I also have a Canon Zoom 105mm camera. For ratings see Consumers Reports magazine in the index under 'cameras, compact.'
Of course, the bicycle itself is a continually evolving, high tech gadget. I sure wish I had shock absorbers for the 1991 Costa Rica trip. To make the roads passable during wet weather (so that vehicles won't get stuck in the mud), they have strewn rocks on the dirt roads. These rocks are imbedded in the dirt, sticking up about one inch average. It makes for rough riding (especially for the hands). I had to keep a minimum speed up, and lower the tire pressure to 35 psi, to keep it tolerable. A friend encountered the same problem in Argentina.
What gadget did I still need? I get a craving on a long trip (over a month) to use a computer. Some are now only 3 pounds for a laptop (with full size keyboard). Much lighter are PDA's without a keyboard and you use a stick (like a pencil) for entry. The best one from my research is 3Com's Palm Pilot 3 or 5. The 3 uses AA batteries that last 2 months average. The 5 is the most Mac like and has better features, but you need special batteries.
Caution: my policy now is avoid riding at night on a trail or dirt road unless I have been there before. I've seen trails and dirt roads suddenly end at a cliff edge without notice because of washout. In the third world, they often don't bother with warning devices. Your bicycle light shining on nothing is not very noticeable. (This is different than for car lights). Also, as I found out the hard way (crashed), it is difficult to see ruts and surface conditions at night, so you are more likely to crash.
In Turkey in 1998, I took a picture of a 30 foot sudden vertical drop (due to washout from the previous rainy season) on a two-lane dirt road as you go forward, with absolutely no barricades or warnings. The locals all know they need to take the detour so why bother with warnings?
For the countryside, the best place to put the head lamp is on your head. A bicycle light attached to the bike riding on dirt will just vibrate wildly, making it difficult to see anything, and won't be anywhere as bright as a focusable headlamp shining at the point that your eyes are fixed on. (Total output may be the same for both lights, but is wasted by the broad-beam bicycle light.) The headlamp I would have recommended is no longer made since about 1990. However, Petzl from France has hit the American market recently with many models. It's available from REI and other mail order houses.
For slow bicycling or night hiking, you want a broad beam. You want a still wider beam for camping or nighttime bike repair (and maybe use of the dimmer bulb). It also keeps your hands free. For most bicycling, focus at infinity. Slight movements of the head place the beam exactly where you want it. The narrowness of the beam is not a disadvantage since your eyes cannot look everywhere at once, anyway. If an oncoming car forgets to dim, shine momentarily into their eyes. They dim the lights. (Unlike in the USA, the majority of cars in Costa Rica dim the headlights for both bicyclists and pedestrians on the highway.) Riding on dirt at night means brightness is required, and nothing beats the brightness per weight factor of a lamp focused at infinity.
A great rear/side light is a strobe light (Nashbar #BN-SL from Bike Nashbar 800/627-4227). It is very light weight (5 oz with battery) because it uses only one C battery (lasts 16 hours). The electronics raises the voltage (as for camera flashes), and it flashes every second. I attached it under the strap holding the left side of the sleeping bag, so the light was visible front and rear. Many car drivers seemed to be fascinated by the light, and slowed greatly. This lite is no longer available, but there's a substitute: A strobe lite from REI (categorized as "emergency") but it is heavier and takes one D battery.