http://adoptaussoldier.org is a great organization through which you can adopt active duty deployed soldiers to write letters and if you want, send packages (and they give recent, updated addresses). i used to volunteer for them.
soldier's angels is another great organization, though they require that you make a commitment of sending packages as well as letters on a regular basis.
anysoldier.org is a great site but it's more for adopting entire groups of soldiers and also tends to emphasize packages (though some soldiers just ask for letters).
unforunately you cannot mail cards to "any soldier," and as far as i know that goes for deployed soldiers as well as soldiers recovering at walter reed. you MUST have a particular name. in the case of sites like anysoldier.org, the package or letter is addressed to the leader of the group of soldiers who will then distribute them to any soldier in the group.
http://www.snopes.com/politics/christmas/walterreed.asp
if you're not interested in doing this on a regular basis and just want to do something nice as a one-off thing, cup of joe for a joe is a great program to buy coffee for soldiers and you can include a note/e-card with the coffee. sometimes you get responses back.
http://www.greenbeanscoffee.com/coj/