The Extended Info Plugin
The Extended Info Plugin shows a summary of information
about the current ontology:

The information includes:
- The total number of terms in the ontology
- The number of descendants beneath each root of the
ontology
- A breakdown of number of terms with 0, 1, 2, 3 and
>3 parents in the ontology
- The number of terms in each user-defined category
- The percentage of terms that are defined
- The amount of free memory (as reported by the Java Virtual Machine; on
some platforms this percentage is always fairly high, even when there's plenty
of free memory)