If your Magento products are not showing up in frontend category pages, that can be because one or more of the below points are not done correctly.

The products must have following set correctly to appear in the category pages in frontend. You can check all the below things in Manage Products screen of any individual product.
– Status must be Enabled (under General tab)
– Visiblility should be Catalog OR Catalog, Search (under General tab)
– Stock Qty (quantity) should be greater than Qty for Item’s Status to Become Out of Stock (under Inventory tab)
– Stock Availability must be IN STOCK
– Category should be assigned (under Categories tab)
– Website must be assigned (under Websites tab)

Clear the cache, re-index the indexes related to product (Product prices, Product flat data and Category products) and everything should work as expected.

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If you are unable to see Product Images in frontend category pages, check this blog post to resolve it: http://ka.lpe.sh/2013/02/26/magento-cant-see-product-images-in-category-page/