var products = this.ProductService.GetAllProducts(categoryId,
manufacturerId, 0, null, null, null, productName,
false, int.MaxValue, 0, null,
ProductSortingEnum.Position, out totalRecords);
you need to set the categoryId to 0, manufacturerId to 0 and productName to "" (empty string)
and then u will get a List of all the products
the getAllproducts is using Nop_ProductLoadAllPaged StoredProcedure
var products = this.ProductService.GetAllProducts(0,
0, 0, null, null, null, "",
false, int.MaxValue, 0, null,
ProductSortingEnum.Position, out totalRecords);
the Administration/Products.aspx is listing all the products in a grid, follow the code there