The ones I sing most often (except at Christmas time) are Lili Marleen, Muss I Denn, Du Liegst Mir Im Herzen, Die Gedanken Sind Frei and Die Lorelei.   

You should be able to find the words to those in the DT or forum, I think. Die Lorelei, Die Gedanken and Du Liegst Mir Im Herzen are also in Rise Up Singing, I think.

At Christmas time I add O Tannenbaum!, Es Ist Ein' Ros' Entsprungen (Lo, How A Rose E'er Blooming), Still Still Still and of course Stille Nacht.

All of these seem to be commonly known among German-speaking folks.