Chosing a search engine may be a rather complex and purpose driven decision. The feature glossary which is appended will help you making your own check list of features. We recommend that you contact the vendor to verify whether the features that you critically need are or not included. The next logical step is to download the most promising engine for a free trial period. If possible, build your entire application around this engine to test is thoroughly. In a second pass, you might want to reconsider other engines, in light of this first experience.
There are other search engines, such as the ones provided by OCLC and SIRSI which are not specifically designed form web browser clients, but are powerful tools that must be also considered for an electronic library which will have to organize massive volumes of data.
Before you purchase an engine, ask whether there is an AT&T rate which has been negotiated. This is the case, for instance, for Fulcrum.
In the future, it would be desirable to evaluate search engines with calibrated benchmarks. Please, send your engine evaluations and suggestions to improve and update this document to firstname.lastname@example.org.