Let me try to give you a hand. Here is a high level overview
Transporter: The key thing that distinguishes the Slim products is that they are designed to be network devices that live on Ethernet and/or WiFi networks. This means that they have the ability to "talk" back to the computer - more specifically they offer remote access to the hard drive so you can select songs from whatever room you happen to be in without going back to the room in which the computer is located. (You can also pause the music, skip, repeat, initiate shuffle etc through the remote.)
The Transporter is a massively upgraded "audiophile" version of the Squeezebox. Sean Adams, the CEO/Founder of SLIM is the designer of both units. SLIM was recently purchased by Logitech. SLIM maintains a very active user forum on their site.
Modwright Transporter: The analog output stage is heavily modified by Dan Modwright, one of the premiere modders in the country. Functionally it is the same as a stock Transporter. Theoretically (I haven't heard it) this should elevate the Trsnsporter to the same level of refinement as the best USB DACs while maintaining all the (unique) benefits of a network device - at a lower price.
You should also be aware of Wayne Waananen at Bolder Cables who did a lot of the pioneering work on Squeezebox modification, elevating those units to an "audiophile" level though they are not constructed to the standards and functionality of the Transporter.
Wavelength USB DACs: In contrast to the SLIM devices, the Wavelength units are connected to the computer via USB. This means that you must be at the computer to control the song selection and transport functions. Theoretically USB cables are limited to 15' in length after which a repeater is needed. This is because USB carries 5v to power the device it connects to. This limitation can be worked around by using a Opticis Fibre Optic USB cable which requires a power supply at the receiving end since fibre will not carry a 5v signal.
What distinguishes the Wavelength products is that they go directly from USB to I2S, skipping the SPDIF stage. I2S is an electrical serial bus interface standard used for connecting digital audio devices together. It is a much more robust format then SPDIF that does not induce jitter. Wavelengths are designed and built by Gordon Rankin who is one of America's premiere analog audio designers (pres and amps) Gordon is a frequent poster on Audio Asylum's PC Forum and also has a basic but very helpful site addressing this technology.
You will see more and more USB DACs in the market (Scott Nixon was the first, Paradisea, Benchmark and Apogee are others).
Empirical Audio: Steve Nugent is an ex-Intel engineer who became known for his efforts on eliminating the jitter inherent in the SPDIF circuitry. He also mods some gear to go USB to I2S, and this year introduced his own USB DAC.
At least on the Mac side, USB and Ethernet/WiFi DACs can share one database - meaning that they can both access the same library - simultaneously.
Steve is a frequent poster on both the Gon and Asylum, he goes by the handle audioengr
Hope this helps - keep reading.