Charity Shops  a.k.a Thrift shops here.

Goodwill  
 Generally very good, although venues vary. Can get everything from lamps, coffee makers  to clothes etc. 
Very reasonably priced and stores have daily specials. Can be found under Goodwill of Greater D.C.
http://www.goodwill.org/


Unique.  Large warehouse sized thrift shops. Slightly more expensive but huge. Google locations. Certain days cheaper and loyalty cards!
www.imunique.com/

Salvation Army. Less of them but again great, for a bargain

Fort Myer thrift shop on the Military Base, Arlington.  Large supply of anything and everything. 
www.fortmyerthriftshop/org.  


Try using YELP  on line, to get reviews of shops, services and restaurants.


Donations;
Thrift stores are always grateful for donations, however some items cannot be accepted. Check for details.
VA Veterans will collect clothes and household donations. Great to use after a Spring clean or repatriating. VA Veterans pickup

You will come also come across small independents.These can be more like consignment stores, which are  similar to used clothing agencies. The donor gets a % back from the sale. It will be apparent as prices are higher than a donated free store. The proceeds are often for a local charity or Hospital. There are also small independents who are conventional Charity shops as we know them.