Teleconverters                                       Canon 1.4x &  2x teleconverter reviews here
Nikon TC-14E 1.4x Teleconverter
Possibly the finest teleconverter available. The TC-14E exists in two guises, the original and the newer MKII version. There are just cosmetic/build differences between the two, the MKII being a lighter weight design and matches the finish of latter Nikon AF-S lenses.
Being a 1.4x teleconverter, you will lose one f-stop of light.
In my experience this teleconverter is optically invisible and I am more than happy to have this between camera and lens for much of the time. Auto Focus when combined with an f4 lens and the Nikon D2x is rapid and reliable.
The TC-14E Will only allow auto focus upon lenses equipped with internal focus motors such as AF-S and HSM. It will not physically attach to non AF-S lenses without a modification, therefore this teleconverter will need to be modified to work with Sigma HSM lenses. Details on this modification can be found HERE
Nikon TC-17E 1.7x Teleconverter
Nikon's latest teleconverter. The TC-17E gives an unusual magnification factor of 1.7x at a cost of one and a half f-stops. In use with my 600mm f4 and Nikon D2x, massive effective focal lengths are involved. Focal lengths exceeding 1000mm present all sorts of additional problems for the photography, notably requiring rock steady support and suppression of mirror slap. At these magnifications atmospheric pollution will also come into play. Perfect long lens technique is required to use this teleconverter with the big lenses.
Obviously, I tend not to use this teleconverter unless absolutely necessary and light conditions are favourable. Auto Focus when combined with an f4 lens and the Nikon D2x is acceptable but not rapid. From all accounts this teleconverter is a considerable step up from the Nikon TC-20E teleconverter in terms of quality of results, but some of this 'quality improvement' will be down to it providing less magnifications and less dependency on lens and stability.
The TC-17E Will only allow auto focus upon lenses equipped with internal focus motors such as AF-S and HSM. It will not physically attach to non AF-S lenses without a modification, therefore this teleconverter will need to be modified to work with Sigma HSM lenses. Details on this modification can be found HERE
Teleconverters are a wonderful way to increase the reach of your set-up without resorting to buying a new lens. Even with large 500 or 600mm lenses, a teleconverter is still often needed for many bird photography situations, not least to get around the problem of having a small bird fill the frame to a decent size with the problem of quite long minimum focus distances with long lenses.

You don't get something for nothing, and with teleconverters you will lose light. A 1.4x teleconverter will lose you one f-stop, a 2x will lose you two f-stops. Losing this light has an impact on many camera's ability to auto focus as most cameras do not like to auto focus when presented with less than a combined lens + teleconverter f# above f5.6, most Canon camera will totally refuse to auto focus when presented with worse than f5.6, Nikon based cameras tend to struggle onwards from f5.6 with diminishing performance.

By the way, don't think because you stop down on your lens, so that the combination is worse than f5.6, it will cause auto-focus to stop working. A camera's auto focus system only ever sees your lens at it's widest aperture (lowest f#), as the lens aperture closes down only when the shutter opens to take the shot (that's why the image is always nice and bright in the viewfinder even when you've stopped down to f32 or the like)

Personally speaking, I would warn against using a 2x teleconverter unless you are really up against it. A 1.4x is by far the best compromise for most.
Kenko Pro300 1.4x Teleconverter
Once upon a time the 'law' said you had to have a teleconverter matched to your lens, that mean't having to buy Nikon teleconverters for Nikon lens, Sigma teleconverters for Sigma lenses and so on ... today it's not important. The Kenko pro300 range of teleconverters proves that third-party teleconverters can be a hair's breadth away from the performance of the very best around. The beauty of the Kenko range is that they are so friendly with so many cameras and lenses...  they seem to auto focus with anything.
What's more, these teleonverters are dirt cheap compared with the alternatives.
This is the 1.4x version that I can fully recommend, as I have used it extensively and still use it from time to time as it is the only teleconverter that will provide AF and VR with the Nikon 80-400mm VR lens.
Kenko Pro300 2x Teleconverter
I no longer own this teleconverter but used it for some shots with a Fuji S2 + Sigma 500mm EX HSM. Given good lighting, it could produce decent results but I still think it's not quite in the league of the 1.4x version (even accounting for extra magnification and any stability issues).
Kenko also make a 3x version of this range... I'd steer clear if I were you and you'd better have some perfect glass in front of it if you do buy one.
NIKON Teleconverters
KENKO Pro300 Teleconverters
The Canon Teleconverter Trick

Canon digital slr cameras such as the 10d and the 20d will cut-off auto focus when they detect a f5.6 lens in addition to a teleconverter.

It's quite simple to get around this by fooling the camera into thinking there is no teleconverter attached and continuing to auto focus by a simple trick.

Just place a thin strip of insulating tape over the three pins on the lefthand side (looking at the lens side of the converter). No damage will occur to the camera or teleconverter. Auto focus performance will not be quite as good as f5.6 but when you are using a lens such as a Sigma 500mm f4.5 with a 1.4x teleconverter, the amount of light reduction is minor and auto focus will still be very good.

When you try this on a f5.6 lens such as the Canon 100-400mm L  I.S. USM in combination with a 1.4x teleconvereter, auto focus will generally be poor.


Birding Top 500 Counter
The photo on the left (click to show at full size) gives you an idea of how much difference a teleconverter or extra focal length will make to the image in the viewfinder