The more common forms (modelos) that need to be filed for an SL are:
Form 111 (quarterly): Withholding tax (IRPF) due for the company's employees and for any invoices received that have withholding tax retained. These are generally invoices from self-employed people (autónomos), including notaries. See sample Modelo 111. Prior to 2011, this was Form 110.
Form 190 (annual): Annual summary of withholding tax.
Form 115 (quarterly): Withholding tax from rental of business premises.
Form 180 (annual): Annual summary of withholding tax from rent.
Form 303 (quarterly): Net VAT tax due. This is usually equal to the VAT tax received minus the VAT tax paid. If you have received more than you have paid, then you need to pay the difference. If you have paid more than you have received, then at the end of the year, you can carry it over to the next year (compensar), or have it returned (devolver). See sample Modelo 303. Prior to 2009, this was Form 300.
Form 390 (annual): Annual summary of VAT tax.
Form 347 (annual): Annual summary of operations with other companies. A report listing all companies that you do business with, where the total invoicing is over 3000€.
Form 349 (quarterly): Declaration of intracommunitary tax.
Form 200 or 201 (annual): Corporate tax form.
Form 202 (3 times per year): Estimated tax payments for the current year, based on corporate taxes paid last year.