How can we insert a dynamic value in the Email Body of an Email Template in Salesforce?

Yes, you can use "VisualForce Email Template" by this you can pass dynamic values in Email template.

Thanks

Here is example;

<messaging:emailTemplate subject="List of opportunity" recipientType="User" relatedToType="Account">
<messaging:htmlEmailBody >
Hi,<br/>
Below is the list of opportunities for your account {!relatedTo.Name}.<br/><br/>
<c:OpptyList AcctId="{!relatedTo.Id}" /><br/><br/>
<b>Regards,</b><br/>
{!recipient.FirstName}
</messaging:htmlEmailBody>
</messaging:emailTemplate>

Thanks