Magento add custom block to product page

Depend upon the position where we would like to insert your custom block you can insert your custom block.

Below is xml to insert custom block into product page

<catalog_product_view>
            <reference name="product.info.extrahint">
                <block type="tag/product_list" name="product_tag_list" before="-" template="tag/list.phtml"/>
            </reference>


            <reference name="product.info">
                <block type="catalog/product_list_related" before="-"  name="catalog.product.related" as="related_products" template="catalog/product/list/related.phtml" >
                        <action method="addToParentGroup"><group>detailed_info</group></action>
                </block>
                    </reference>
    </catalog_product_view>