Index of ecological niche overlap
Pianka's index of niche overlap is defined in his papers from 1973 and 1974, as:

$O_{kl}=\dfrac{\sum_i^n{p_{il} p_{ik}}}{\sqrt{\sum_i^n{p_{il}^2} \sum_i^n{p_{ik}^2}}}$ 

where $O_{kl}$ is the resource overlap between species $k$ and $l$, and since the index is symmetric $O_{kj} = O_{lk}$. $p_{ib}$ represents the proportion of resource $i$ that is used by species $b$. The index is a modification of one presented in MacArthur &amp; Levins (1967).

As apparent from the formula, the only thing you need to calculate the index are estimates of resource use from the resources that both species depend on. The tricky part can be to determine how you calculate proportianal use of all resources. 

I haven't used the index myself, and cannot really say anything about how common it is. I have seen references to it though. Pianka (1973) use it to study overlap between lizard species, and how the distribution of niche overlaps differ between habitat types. I haven't seen any statistical tests on it, but you should be able to use resampling or bootstrap to determine the uncertainty in the index.