I should qualify that a bit--I speak French fluently, which makes other Romance languages relatively easy for me, and I'd had a few after-school Spanish tutoring sessions as an elementary-school student and listened to my parents' set of Italian-language records sometime during middle school or high school--but the principle holds true: reading English literature in translation and and classics in other languages that one has first read in English translation can be a cheap, enjoyable, and effective immersion experience. It won't make you a fluent speaker of a language by itself, but it will expand your vocabulary and give you a feel for the grammar quickly and painlessly.
This obviously works best for languages that are related to English (or to whatever other languages you can speak or read). With other languages a lot of tedious dictionary work will be needed, and that will be off-putting to all but the most patient and motivated language learners.
I also use literature in translation to help my children learn French and Spanish. Many popular English-language picture books have been translated into these languages, and kids often find it easier to pick up vocabulary from a book whose story they already know than from an unfamiliar book. My three-and-a-half year-old son will often bring me two versions of a story at bedtime and ask me to read them back-to-back. Some even come with a CD, which may be particularly helpful to parents who don't know the target language or don't pronounce it well.
For very young children or those just beginning to learn a language, I recommend translations of Goodnight Moon, by Margaret Wise Brown (illustrated by Clement Hurd) and Bill Martin Jr.'s books Brown Bear, Brown Bear, What Do You See?
For somewhat older children, favorites include Ludwig Bemelmans's Madeline books (as good in French as in the original English) and Jean de Brunhoff's Babar stories
My son particularly enjoyed Deborah Guarino's more recent book Is Your Mama a Llama (illustrated by Steven Kellogg) in a Spanish translation by Aida E. Marcuse
There are many other children's classics and recent bestsellers available in French, Spanish, and other languages. There are also what I call "corporate" books that are terrible as literature but may be useful for the purpose of learning a new language; these include English and Spanish versions of the Dora the Explorer books
Teenagers and other more advanced students may enjoy reading The Adventures of Pinocchio in Carlo Collodi's original Italian