R from the gym is a Mac user. He referred me to DEVONThink and Yojimbo, both applications only for the Mac platform, as possibly similar systems to what I have in mind.

Well, they are not. The bottom line is that they are everything-in-one-place data clearinghouse with limited pre-defined data types. Both support full-text search and tag-based search, but DEVONThink supposedly goes further.

DEVONThink's "AI" makes automatic association of keywords, and finds items that do not have the words/tags you supplied, but are related none the less. I am not sure how this automatic association is done. Is it done by analyzing data, or does it make use of some thesauri? Either way, I personally think calling it "AI" is a bit of a stretch. R told me to read this article, which shows how an author uses this software.

DEVONThink can work as a Web server to publish your data and optionally let others to contribute also. Nice feature.

Evernote is also in the same line, but with support for multiple platforms including iPhone and Palm Pré, it looks more sophisticated.