A JMC2 Suprema Project, 2004.
Cryptographic webs of trust aren't quite fully decentralised as a network of keyservers hold a complete view of graph. However, anyone can verify that the graph is correct and thus there is no need to trust keyservers. At worst the keyservers could refuse to admit to the existence of an edge in the graph and so reduce the level of trust that would otherwise be afforded to a node. However, any person can check the correct functioning of a keyserver and produce a proof of its compromise if it fails to act correctly.
By far the most extensive web of trust is the PGP keyserver network. This allows each edge to be assigned one of five levels from "complete trust" to "only checked casually". How this information is processed is left up to each client as each client can download as full a view of the complete graph as it wishes. Often a system of maximal network flow is used like the Advogato trust metric.
It's also worth noting that the notion of "trust" in the context of PGP webs is very different. Trust in this case refers to the confidence that a given key is controlled by the person that it says it is, not that the given person is in anyway trustworthy.