Another tutorial has just been added on macro photography:
Macro Extension Tubes & Close-up Lenses
Most of the content compares and contrasts the two options for adding magnification. Several examples and calculators are provided. It's meant to complement the earlier tutorial on macro lenses, and is a lot shorter.
As usual, comments/typos/suggestions/etc are all welcome. I'm still in the process of editing it so there may be some minor changes/fixes over the next few days...