What is one method for implementing Visualforce mashups?

Using a Visualforce iFrame tag is a method that effectively facilitates the implementation of Visualforce mashups. An iFrame allows developers to embed content from external sources directly within a Visualforce page. This approach is particularly advantageous for displaying data or functional components from different applications or systems while maintaining the Visualforce framework's styling and features.

This method supports the integration of various web applications seamlessly, allowing for the combination of different functionalities into a single user interface, which is essential for creating rich and interactive user experiences. Using an iFrame also helps in managing cross-domain data effectively by leveraging the capabilities of the browser to display content from other domains.

In contrast, the other methods listed do not offer the same level of integration or are outdated technologies that do not align with modern web standards or Salesforce best practices. For instance, traditional HTML pages do not leverage the features specific to Salesforce and Visualforce, and both Java applets and Flash objects have fallen out of favor due to security issues and lack of support in modern browsers. Thus, utilizing iFrames is the optimal approach for implementing Visualforce mashups.