Materials that may be used without being koshered—are materials which do not absorb and do not emit.
The most common example of this is glass. This holds true, even if the glass is used for cooking. Thus, even Pyrex and other such substances do not absorb. (It should be noted, regarding Pyrex, however, that this is not universally agreed upon.)
(Regarding Corningware, click here.)
As well, colored glass does not absorb. However, it is important to note that glass with a painted trim—may absorb. Additionally, Annie Glass has its own ruling.
As is well known, most Ashkenazim do not follow the leniency on glass.
To see a comprehensive list of various utensils and the methods of koshering them, click here