Eclipse, using the Web Standard Tools (WST) Project can provide a very nice environment for editing .zpt or .pt files (or any other XHTML or XML file). Of course, it goes beyond the "html" and also provides integration for editing css and js files (even though at a somewhat basic level).

To install the WST you need to activate the update dialog and search for new items to install in the Callisto project (assuming you're using a default Eclipse download from eclipse.org). You'll have to select:

• Graphics Editing Framework (GEF)
  
• Eclipse Modeling Framework (EMF) Runtime
• XML Schema Infoset Model (XSD) Runtime
  
• Java EMF Model
  
• Web Standard Tools Project

After installing all these, you'll have to configure the XML content type to include the .pt and .zcml extensions: Go to Window > Preferences > General > Content Types > XML and add those new extensions.

Eclipse 3.3

For Eclipse 3.3 (Europa), you need the following combination of plugins:

• Eclipse Modeling Framework (org.eclipse.emf.common)
  
• Eclipse Modeling Framework Runtime + end user tools
• EMS Service Data Objects (org.eclipse.emf.ecore.sdo)
• Graphical Editing Framework
• Web Standard Tools
• WST Common UI
• XML Schema Infoset Model (XSD) - org.eclipse.xsd
• XML Schema Infoset Model (XSD) - org.eclipse.xsd.edit