What are the different tinning techniques for PCBs, and what are the advantages of one technique over the other?
The easy answer is that you should look at the options your board house offers and then research that.
Common options I've seen are:
HASL, Hot Air Solder Leveling: They bascially coat the entire board in solder and use a air knife to blow the excess off, it's cheap but it can leave a slightly uneven surface, which might mean something if you are doing BGAs.
OSP, Organic Solderability Preservatives: A very thin coating that keeps oxygen away and evaporates when soldering, it looks as though there is just bare copper on the board.
This page has a good description of these and other finishings: http://www.multicircuits.com/pcb/tech/surface_finishes.html
Immersion silver seems to be getting popular for RoHS boards. It gives a very flat surface, and is fine if boards are soldered quickly. Some companies use immersion gold, even on low-cost boards. Silver can oxidise over time, which apparently can affect solderability, so gold might be better if boards are to be stored. I've had no problems hand-soldering boards finished with both immersion silver and immersion gold (lead solder), and neither has the company I use sometimes for surface-mount board assembly, using lead-free solder paste.
Gold plating has been used on connector pins for years, and has never caused soldering problems.
Most of the work that I did in recent years have called for ENIG - Electroless nickel immersion gold. The plating comes out nice and flat, which is not the case with HASL. It also looks pretty, but that's a subjective thing. :)