There are many needs that the troops have that are not met by the "usual methods" like sending mail to "any soldier" (I'm not disparaging those methods, by the way; they too are useful). I'd like to spotlight four of them, because there are some unusual projects that are very helpful.
Phone Cards for Wounded Soldiers
The combination of excellent protective gear (including universally-available body armor), unparalleled combat care, rapid medical evacuations, dedicated and experienced trauma surgery units very near the battlefield, and the advances made in modern medical care mean that many soldiers who would have died in past wars are instead wounded, sometimes terribly so and in ways that require long convalescence. This is a good thing.
Unfortunately, one of the less good consequences is the long convalescence, and the fact that the Army doesn't pay for long-distance calls made by the convalescing soldiers. One of the most in-demand items among the wounded soldiers is phone cards to call home with. You can send them to Walter Reed Medical Center at:
Medical Family Assistance Center
Walter Reed Medical Center
6900 Georgia Avenue, NW
Washington, DC 20307-5001
Hugh Hewitt has details.
Baghdad Hobby Club
It's often not well-known to civilians, but fighting a war is usually boring. Most of the time, you are waiting to do something, and in between periods of waiting, you are terrified. Having "episodes of normality" - including mail from home talking about the mundane details and progressions of daily life - is a link to sanity, to a promise of getting home to what you miss most. The Baghdad Hobby Club is a group of soldiers "very interested in gaming, scale models, railroad, and radio control to pass their time in Iraq". Sending hobby gear can help the soldiers keep their sanity and their focus. You can send items to:
Sgt Dean Flyte
Baghdad Hobby Club
APO AE 09342
Winds of Change has background and details.
Fallen Heroes Last Wish Foundation
The Last Wish Foundation provides for the education of children of service members killed in Iraq. There are other such foundations, and a list of worthwhile charities in a similar vein can be found at Roll Call's family support page.
Spirit of America
A big part of the mission in Iraq is to build Iraq up to the point that it can stand on its own as a free nation. Because of decades of deprivation under the Ba'ath tyranny, everything is lacking. Spirit of America provides goods and services to ordinary Iraqis and Afghanis to help them rebuild their country. For example, providing tools to tradesmen or library books to schoolchildren.
Please feel free to leave other suggestions in the comments.