For a consulting assignment last year I had to survey the field of textbase
software, text/wordprocessing indexing software and document imaging
software.  At that time, the best and most flexible INDEXER of existing
files was Isys, and the best (but simultaneously most annoying) textbase
where you could throw in anything and index it in a lot of genuinely useful
ways was Ask Sam.  ZyIndex came in someplace in between, and a shareware
thing called dTsearch wasn't bad (esp. because it was shareware and you
could try it out for free).

Anybody who wants to know more about any of these or the ones not mentioned
here, feel free to email me.