All large commercial aircraft carry these black boxes, which record sounds in the cockpit as well as operating parameters (altitude, airspeed, engine thrust, etc.) on continuous loops.  The storage media (magnetic tape in the older ones, solid state memory in the newer ones) are encased in hardened stainless steel or titanium capsules that are built to survive incredible forces (up to 3,400 Gs) as well as fires, explosions, and submersion in water as deep as 20,000 feet (6,150 meters).  They contain radio beacons to aid in their location in the event of a crash.

In the U.S., the National Transportation Safety Board (NTSB) has the job of extracting and analyzing the data from Cockpit Voice Recorders (CVRs) and Flight Data Recorders (FDRs) and using them in conjunction with other information gained in its investigation in order to determine the cause of the accident.  Because of the nature of the recent incidents, the FBI is also involved.  Links to the NTSB site, the FBI, and several other relevant sites appear below.  All links have been updated and are current as of the date of this Tidbit.

Links:
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Description of Cockpit Voice Recorders (CVRs) and Flight Data Recorders (FDRs) from the U.S. National Transportation Safety Board (NTSB).

Description of CVRs and FDRs from Honeywell, Inc., which merged with Allied-Signal and 1999 and manufactures CVRs and FDRs, as is the source of the CVR pictured above.

Origin and early history of the "black box," which was invented in Australia, from the Australian Defence Science and Technology Organisation (DSTO) (includes a picture of the original 1958 "Flight Memory Recorder" and interesting technical details).

"A Better Black Box," by Mark Alpert, Scientific American, September 2000.  An excellent, easily understandable discussion of how they work, what they do, etc.

"How Black Boxes Work," on Marshall Brain's How Stuff Works site.  Another excellent source; includes photos of FDRs and CVRs recovered from recent crashes.

Links to all CVR transcripts available on the Internet, from aircraft crashes between July 6, 1962, and July 4, 2001; also some recordings (downloadable as .wav files) and other interesting information about CVRs, FDRs, and aviation safety.

Answer to the classic question: "If aircraft "black boxes" are indestructible, why can't the whole plane be made from the same material?"

Yahoo EgyptAir 990 crash information page.

Google's links for information related to the Sept. 11 terrorist attacks on the U.S.  See, also, CNN.com and The New York Times.

Information regarding the September 11 incidents from the FBI.