How do I get to/from Dulles Airport in Washington, D.C.? I've reviewed the options on the website and it's hard to tell what's actually the combination easiest/cheapest way.
We're staying near Eastern Market if that makes a difference.
Washington Flyer bus
If you are staying near Eastern Market, I would generally recommend the Washington Flyer bus, which is an express service to the West Falls Church Metro station (~20 minutes), where you can board the Orange Line to New Carrollton and ride in all the way to Eastern Market (~35 minutes). The bus is reliable and generally clean, and your bags will be stored a separate compartment below the cabin. Washington Flyer does not depart from the main shuttle bus area; it has its ticket booth and station just outside Arrivals door 4 at the airport.
The fare is $10.00 one-way, $18.00 round-trip per person, plus $3.50-4.60 for Metro depending on when you travel. In my opinion, this is usually the best trade-off of cost versus time.
A cheaper option is to take the WMATA Metrobus 5A to Rosslyn (~45 minutes), then transfer to the Blue or Orange Line and ride out to Eastern Market (25 minutes). You can also stay on the 5A all the way to the end of the line at L'Enfant Plaza and board the Blue or Orange from there. This will take longer and mean more time on the bus, but if you arrive during the morning rush hour, it may be more civilized to keep your seat on the bus than to contend with the crowds at Rosslyn.
The 5A is an express service that makes only one or two stops before Rosslyn; however, it is an ordinary public bus, so at best it will have overhead racks for small bags, and it is often crowded. On the other hand, the fare is only $6.
You can pre-negotiate a pickup from a taxi company OTHER THAN the airport taxi. The Washington Flyer Taxi has a monopoly on service to the airport and its rates are fixed (about $75 to Eastern Market plus surcharges and tips). You can privately arrange for a different taxi or car service to pick you up, however, just as you might ask a friend or family member to pick you up. You can avoid the hassle of the taxi stand and maybe a few dollars as well, and if you come to Washington frequently, may be worth the investment of getting to know and keep a favorite driver.
Another option is to take SuperShuttle, which is a shared van service. SuperShuttle makes the most sense if you are traveling alone to a destination not well-served by Metro, but the rides can take a very, very long time if you are one of the later passengers to be dropped off. From Dulles, I generally only recommend this service for people traveling to the Virginia suburbs.
One-way car rental
Yet another option that makes sense for some is to rent a car from Dulles and return it to Reagan National the same day. Some rental agencies may waive the one-way fee on certain rate codes. If you have a good-sized party or many bags, this is sometimes an option. Of course, at least one person will need to take Metro or a taxi from Reagan National back to where you are in Eastern Market, and driving this route can be dicey during rush hours or if you are unfamiliar with the area.
Can you wait a couple of years? They're extending the DC metro to the airport at the moment... The metro should reach Tyson's Corner in 2013, and the airport a few years later (2018 is apparently the latest date).
If you don't want to wait several years, it all depends how much of a hurry you're in, and when you're going. During peak hours, your best bet is to take the bus between the airport and Rosslyn metro station, and the Orange line between the city and there. Traffic can get really bad on the freeway at peak times, so you can smugly speed past it on the metro. The bus ticket is $6 (exact change only), metro onwards is a few dollars depending on your destination, and the 5A timetable is here. I've done this myself, and I found it fine, even with a moderate size bag.
However, because of the work to extend the metro, evenings / weekends can see a lot of engineering work taking place on the Orange metro line. For those times, you'll be better off taking a bus from downtown to the airport. There are both private and public ones, with differing costs / comfort. The MWAA site lists the various options
From next year, I think the best bet will be bus to Tyson's Corner, then the new metro line into town.
When I was there last I used SuperShuttle. Its dead simple (just walk to their counter) and reasonably priced at about $30, if you are traveling in a group, additional passengers cost $10. Compared to taking a cab by yourself or with one other person, this saves a bundle.
Of course it is a shared shuttle service so it will take a bit longer than a cab ride, but it will get you all the way to your hotel. The only cheaper alternative is to take the bus to where the metro ends and take it into town. This will be cheaper but will likely take longer, especially if the metro does not stop close to your final destination.