I discovered a free, monolingual German dictionary online a few days ago. It’s part of Das Digitale Wörterbuch der deutschen Sprache des 20. Jh., or DWDS. You can access the dictionary, as well as word information and the text corpus which the information is based on, here. I’ve looked up a few words in the dictionary, and while the definitions can at times be quite short, sentences or snippets of text are shown to give the word some context.
If that’s not your cup of tea, but you still want to try a monolingual German dictionary, you might want to check out Langenscheidt’s Großwörterbuch: Deutsch als Fremdsprache. It’s now available in paperback as well as in CD-ROM format.