A review to the basics is never a bad thing, so here we go:

* Yahoo! owns Overture, AllTheWeb, and AltaVista. Yahoo! also owns
and uses Inktomi's technology and database for all the partnering
engines under its wing.
* Google partners with AOL, it is also a major result provider for many
large engines because of its own cutting-edge technology (HotBot,
Lycos, Ask.com, Iwon).
* Ask Jeeves (Ask.com) owns and uses the Teoma (formerly DirectHit)
engine, results and crawler, Excite Europe.
* MSN does not own anybody and uses its own database (since
February, 2005).
* AOL owns Netscape.
* Lycos owns HotBot and Tripod.
* Netscape owns DMOZ.

Here is a static representation of the relationships between search engines. (by Bruce Clay)