Want to update the Lead Source field on a Lead record based on which form the person filled out? You can do that with a Workflow.
Base the workflow off of the Posted Form entity. Then we'll check which form they submitted and update accordingly.
Set the Scope to Organization and keep it set to Start When: Record is created. Here's our workflow with an explanation below.
First check to see if it was a Lead that filled out the form (Posted Form - Lead - Contains data). Then check if it is one of your forms. In our example we used the Newsletter form. Posted Form - Form - Equals - Newsletter. Now do an Update Record step to update the Lead Source Field on the Lead with the appropriate Lead Source for that form. Click the Set Properties button and find the Lead Source field. Choose the source you'd like for that particular form.
On the same level as checking if the Posted Form was the Newsletter form, do another if statement for each Form you have and update the Lead accordingly.