Details

http://www.formassembly.com/

If you are doing more than one simple Web To Lead form then I would strongly recommend this. 

Here is a good article on using Form Assembly rather than Web To Lead.

Documentation

http://www3.formassembly.com/blog/doc/connectors/salesforce/configuration/

Notes

To Prefill data securely, you need the Form Assembly Premier edition at $130/month and then you need to set up the security in custom code in Salesforce. 

http://www3.formassembly.com/blog/doc/connectors/salesforce/prefill/

Depending on your data requirements and security requirements, you can email out the form pre-filled, with the regular edition of FA by just passing the data through the URL - but be very careful with this

Issues

Authentication

Uses Token Authentication (on Professional and Premier Editions of FA). Oh cool they now use oAuth. I would still strongly suggest using a separate Integration login though. 

Using

It is a bit of a dog to use - you have to go into a completely different section to edit the form, edit the themes and set up the connector. 

Dealing with sections is really finicky. You never know where they are and how nested they are. It is so hard to move questions between sections. Use the Move this Question Up and Down links and the Parent Section link to try to control it. And look in the connector to tell you how many sections you have.

I add some CSS to the form builder to clearly see the sections, then comment it out when the form is to go live. (But also use the hierarchy view to see the different sections)

 /*Editing*/
  .section .group {
    border-width: thick;
    border-color: blue;
    border-style: solid;
  } 

NPSP

The trick to working with the Nonprofit Success Pack - NPSP is: 

In my form, I need to create a Contact (which will then create the Account) and an Opportunity. To create an Opportunity, I need to get the AccountID that was created automatically. Luckily the Account that is created automatically has a lookup to the contact. Query the account based on the Contact ID that was created first, then pass that through to the Opportunity.

Things I want Form Assembly to Fix

• Be able to switch from the form editor straight to the connectors and to the logs and back again - just let me get from one place to the other without having to go to the main page. It's sort of there, but not integrated with the Build screen yet. 
  • Eg, I want to be in the Form, the Connector and the Logs in three different browser tabs at the same time. 
  • Even from the Log, I can't get back to edit the connector without going through the menu form again - Open it in a new tab AND let me get back to the connector that made this log. Don't make me hunt for everything.
• Don't ask me do I want to exit the form every time I save - if I want to exit the form I will exit the form! 
• Piping text from question answers into different text areas of the form - eg like what SurveyGizmo or other survey tools allow. 
• Conditional text - but place text in a section with no heading and make the section conditional.
• Be able to use prefill values in long text areas - eg build a sentence or a paragraph based on prefilled parameters or formulas. 
• Be able to create a separate chatter post for each record of a repeating section. 
• Be able to create a chatter post with an @ mention. 
• The ability to have repeating sections go over pages. 
• Open up EVERYTHING from the menu page in a new tab. Open up ALL help links in a new tab. 
• Show section boundaries - they don't need to appear in the displayed form, but they are needed for being able to see where to drag and drop things.
• Be able to copy a section or a page or a selected set of fields into another form - eg if I have set up the CC fields for Stripe, I don't want to have to re-create them each time. 
• Be able to see exactly what was sent along in the POST with the HTTP Connect to debug it. 
• Display images based on URLs passed from Salesforce. 

Nice to have:

• Better themes - eg make one based on the Lightning Design System, or to look like a standard Gravity Form in WordPress - make things look a bit more modern. 
• Better structure to the Menu page. 
• HELP people with Javascript on their forms - HELP people do really cool stuff with your product. 

Cool things

I do forms with multi branding. 

I pass in the brand as a parameter (preferably by a secure Salesforce parameter). 

I hide the form title with CSS

  /* Hide Title of Form */
  .wFormContainer .wFormTitle {
    display: none;
  }

I add a section with a drop down field to accept the brand parameter, then I hide that whole section with CSS. 

  /* Hide custom fields */
  #tfa_87, #tfa_43-D, #tfa_68-D {
    display:none; 
  } 

I add a section containing a new text for form Title and the brand logo, then conditionally display that section based on the parameter. 

And I increase the font size of the new form Title text. 

   /* Enlarge major Headings */
  #tfa_89-HTML {
    font-size:18px;
    font-style:bold;
    padding-top: 40px;
    padding-bottom: 10px;
  }

So then you only have one form to maintain, but it can work for multiple brands.