[reverse chronological order, more or less]
A Genetic and Insertion Heuristic algorithm for solving the dynamic ridematching problem with time windows. Proceedings of the fourteenth international conference on Genetic and evolutionary computation, 2012. Algorithm for "minimizing the total travel distance and time of the drivers and the total travel time of the riders and maximizing the number of transported riders.... Experimentation results indicate that the algorithm can successfully solve the problem by providing answers in real-time and it can be easily tuned between response time and solution quality."
A Mechanism for Dynamic Ride Sharing based on Parallel Auctions. (local copy, in color) Proc. of the 22nd International Joint Conference on Artificial Intelligence (IJCAI), 2011. Application of incentives to dynamic ridesharing with "auctions using a sealed-bid second price scheme." "Our system allows to tradeoff the minimization of Vehicle Kilometers Travelled with the overall probability of successful ride-shares, which is an important feature when bootstrapping the system."
Markets for Dynamic Ridesharing? Case of Berkeley, California, February 2011. Lots of good background information, survey/market analyses. Primary conclusions: (1) potential for new rideshare trips, though may attract current transit users; (2) probably costly; (3) commuters generally unaware of existing (non-dynamic) ridesharing options; (4) users of dynamic ridesharing would do so only occasionally; (5) users would prefer to arrange trips at least the evening before.
A bumpy ride for taxicab sharing: Lower prices aren't enough to get people to pile into the back. Crain's New York Business, February 13, 2011. Several taxi-sharing startups, plus a project of the Taxi and Limousine Commission, indicate that the "critical mass" barrier remains high.
Ridesharing in North America: Past, Present, and Future. Nelson Chan and Susan Shaheen, August 2010. A survey of the development of ridesharing, including updates on current efforts.
Implementing Real-time Ridesharing in the San Francisco Bay Area, June 2010. Comprehensive survey of ridesharing projects, including hard-to-find information such as a pilot test at Genentech's headquarters in South San Francisco and a pilot at the Centers for Disease Control in Atlanta.
SafeRide: Reducing Single Occupancy Vehicles, Jim Morris, January, 2009. A working paper for a potential project in the San Francisco Bay Area. Comprehensive overview of the subject, including references to previous and current projects.
Research Bibliography prepared by Carnegie Mellon Ph.D. student Kursat Ozenc <email@example.com>
A Market Analysis by a team of Carnegie Mellon MS Students contains much current (2009) information about SF Bay Area players and a good financial analysis framework.
Interactive System for Real Time Dynamic Multi-hop Carpooling, Philip Gruebele, September 2008.  Proposes a system in which "Passengers and vehicles are matched without the requirements of sharing a similar destination or of passenger destination being positioned on or near driver route. Passengers can move towards destination one hop at a time..." Apparently no implementation steps (e.g., software) so far.
How I Solved the Energy Problem, Scott Adams, June 2007.  A well-commented-upon blog by the Dilbert author "inventing" the concept.
The Smart Jitney: Rapid, Realistic Transport, Pat Murphy, New Solutions, April 2007.  Dynamic ridesharing with private vehicles reframed as "jitneys." The report suggests more organized vetting of the drivers; getting drivers to put a "Smart Jitney" logo on their cars is a nice marketing touch. The report also includes some ideas about improving law enforcement best ignored. No evidence this proposal has gone beyond the paper stage. See also: Smart Jitney: A Modest Proposal
Empty Seats Traveling: Next-generation ridesharing and its potential to mitigate traffic and emission problems in the 21st century, Stephan Hartwig, Michael Buchman, Nokia Research Center Bochum, February 2007.  Overview of dynamic ridesharing concept, usage scenarios, patent history, brief discussion of Amsterdam and Frankfurt airport projects.
Casual Carpooling – Enhanced, Kalon Kelley, Journal of Public Transportation, Vol. 10, No. 4, 2007 Casual Carpooling – Enhanced (extracted from ). Yet another paper invention of dynamic ridesharing. Emphasis on vehicle-based transponders and "radio frequency identification devices" (electronic tags) to identify passengers and provide accounting.
Car-Pooling – An Idea Whose Time Has Come, Ben Rosen, The Huffington Post, January 2007.  A successful tech venture capitalist describes his initial work (with "a team of software engineers") and market research on a proposed dynamic ridesharing system. His discussions with government officials were not encouraging. (He has since given up)
Auctions Negotiation for Mobile Rideshare Service, Abdel-Naby, Stefano Fante, Paolo Giorgini, circa 2007 (local copy) from . Computer-science based research – "agent-based rideshare system that is accessible via lightweight devices... auction mechanism as a method of negotiation among autonomous and proactive agents...". The important idea in these papers is that it would be possible to organize ridesharing respecting the two following constraints: to provide financial incentives to users, and to avoid direct management of money.
Pilot Tests of Dynamic Ridesharing, Dan Kirshner, December 2006, minor revisions more recently.  A thorough description of one would-be entrepreneur's efforts to implement a system called "Ride Now!" Three different tries were made, the most elaborate involving a Federal grant and the cooperation (and non-cooperation) of local agencies in the San Francisco Bay Area. Describes what worked – and what didn't.
Organized Dynamic Ridesharing: The Potential Environmental Benefits and the Opportunity for Advancing the Concept, Amber Levofsky and Allen Greenberg, Transportation Research Board, January 2001. pdf
Flexible Automobile Sharing Transit (FAST), 2000. Another outline/proposal of the concept. A bit bombastic, but all the features are there. pdf
1982 Automated, Door-to-Door demand-responsive transportation system US 4360875 (Robert W. Behnke)
2001 Passenger Transportation System and Method US 2002/0011940
2003 US 182183: a method of organising a multi-car pool.
2004 US 6697730 (Georgia Tech): an urban transit system employing cellular communication, GPS locating technology, and computers to provide real time command and control of passengers and vehicles.
2004 JP 220396: a car sharing information supply system.
2006 US 7136747 : Method for GPS carpool rendezvous tracking and personal safety verification.