Cassandra Steen has certainly become a well-known artist in Germany after having won an ECHO Award, however I am not so convinced everybody knows her outside her native country.
I discovered her by chance searching for new cool tunes on YouTube and must say that she can match up the level of other more famous American artists! She sings beautifully in both German and English languages.
If you would like to know more about her, look her up on Wikipedia. Otherwise, if you would just like to listen to some of her tracks, check out the below videos (one of them cannot be embedded, so you will need to click on the link to access it):
Another solo song (in English):