Let's start with my personal favorite... wide white mat, simple, high quality, black frame, (photo, thanks to Living etc.) This style looks great with watercolor paintings, photographs, mixed media collages, oil and acrylic paintings in unwrapped canvas (meaning, without the thick, wooden frame), and drawings of all sorts.
These three lovely photos (and the three bellow) were found on the Black.White.Yellow blog (talk about inspiring eye candy!)
Similarly, wide white mat, white frame. Chic, simple, minimalistic, and very romantic.
This style really allows the artwork to stand out.
Another option is no frame at all! This can look great with a nice, thick canvas, or a painting on wood with a wooden box frame nailed to the back. Simple, and stunning. With this style (or lack thereof,) it's all about the artwork. I found this beautiful seen on the Neutral Dwelling Blog.