2 Answers

unusual

A non-standard would be anything falling outside the norm of a given collection (scientific, economic, political, or whatever). Yvonne's answer would be a good "street usage" term, but might not be accurate in normative semantic use, as a non-standard item could be (in statistical fact) not that "unusual" but still be outside the norm (or defacto standard). Depending on how you want to use it as a synonym, a better choice might be atypical. For example, saying that being left-handed is unusual is semantically (that is, implied/actual word-meaning usage) not the same as saying being left-handed is atypical. As there are enough left-handed people in the world, "unusual" wouldn't be the best fit. Oh the joys of the complexities of language, right? :)