Simpleform

It's a D7 port of YAML Form with some extra features.

Example form (YAML file)

label: Example form
description: This is just an example

settings:
  eta: 2 minutes
  requirements:
    - less than 2 minutes of free time
    - enthusiasm
  confirmation_message: Thank you for submitting the form.

elements:

  # Step 1
  personal_details:
    .title: Personal details
    .type: step

    person_details:
      .title: Your details
      .type: component
      .reqired: true
      .component: person
      .overrides:
        first_name:
          .title: Your fitst name
        last_name:
          .title: Your last name

  # Step 2
  company_details:
    .title: Company details
    .type: step

    company_name:
      .title: Company name
      .type: textfield
      .required: true

    company_number:
      .title: Company number
      .type: numberfield

    earnings:
      .title: Yearly earnings
      .type: select
      .options:
        '0_20': Less than £20k
        '20_40': Between £20k and £40k
        '100_plus': More than £100k

  # Step 3
  additional_info:
    .title: Additional information
    .type: step

    source:
      .title: How did you hear about us?
      .type: select
      .options:
        internet_search: Internet search
        friend: From a friend
        other: Other, please specify below
        
    source_other:
      .title: Other source
      .type: textfield
      .required_when: # Module "conditional" is required
        source: other
      .visible_when: # Module "conditional" is required
        source: other