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Beige and tan are most often used in websites simply as background colors. They're commonly seen used in paper textures, grunge sites, or, occasionally, as a solid-colored background. The advantage is that, because they're neutral, they can easily be combined with any number of other colors without clashing.
The websites shown below offer a pretty complete picture of how beige and tan are used in today's website designs. There are twenty five sites included, but please feel free to share more within the comments.