Commerce Fieldgroup Panes is an excellent module that exposes fields into the checkout process. It creates one checkout pane for each fieldgroup of the order entity using Field group module.

This is one of the easiest ways to collect information in the checkout phase, it can be used for many purposes, probably getting customer comments is the most usual, but this screencast also shows how to use information exposed by this module as data for registering users in the checkout process.

These are the basic steps to set up this module and collect information from your customers.

• Create one or several fields in the Order entity type (Store » Configuration » Order settings » Manage Fields)
• Embed those fields justs created in a new Field group.
• Access the checkout settings admin screen (Store » Configuration » Checkout settings)
• Drag & drop your fieldgroups exposed as Checkout panes to the correct Checkout pages.

Other great thing of this simple and useful module is that all the information stored in the Order entity can be used afterwards with Rules and Views, the screencast below will also show how to use this collected information for creating new user accounts.