Specialized Indexes

A number of individuals and groups have taken it upon themselves to set up databases of all the pages they find or are told about that deal with a specific topic. Some are general, like this one for botany.

Internet Directory for Botany - all links

Others are more restricted, like this one for butterflies.

Electronic Resources on Lepidoptera

NOTE:
Some of these are searchable by keywords which may or may not result in a hit on the topic you are interested in.