Legacy.com and PoemHunter.com contain a variety of poems that deal with grief and mourning. "Dulce et Decorum Est" and "On His Deceased Wife" are two examples.
Leacy.com lists 25 poems that deal with losing a loved one as well as death and loss in a more general sense.
PoemHunter.com suggests 494 poems under the subject of Grief, in a list that spans Shakespeare to Shelley. The list also includes some contemporary poetry.
A page on the Academy of American Poets' website includes an article that discusses grief in poetry. The article includes examples of poems written about the loss of a loved one, and it features a list of suggested poems that cover the topic.