Artifacts are mostly used for researching but can also be used in several recipes exclusive to Thaumcraft 2. They are also the best items to use as boosters for research, with each item providing a larger boost for it's own type and exactly half (or rounded down) boosts for other types.
All values listed below are without Bookshelves or Brain-in-a-jar.
While Zombie Brains themselves are not considered artifacts, for the purposes of research they are equivalent to uncommon forbidden artifacts.
A few vanilla and other thaumcraft items will also be listed for comparison.
Note that tainted log is tainted and netherrack is forbidden The bonus values listed above also apply during the fragment research phase and theory research phase, and again the value is greater when using artifacts of the same type as the fragment/theory, fragments have a very low chance of success by default so using a few artifacts to boost is probably a good idea.
Most of the vanilla items are classified as lost, but a few are considered as forbidden such as ender pearls?, ghast tears?, netherwart?, blaze rods?, and magma cream?.
Also note that bookshelves, arcane singularities, and diamonds have the same value, so it's probably better to use bookshelves over the other two as they are a renewable resource, zombie brains are also a good choice, as even when used for research other than forbidden they still provide a 10% boost.