I know this question is pretty old but I wanted to post this answer in the hope that it will help someone else looking for an answer. This is one of the top posts that came up while I was looking for a solution.
Adobe has free web app called Creative Cloud Extract that gives you read-only access to PSDs. Click here to learn more about what you can do and how to use it. You just need to sign up for a free Adobe ID if you don't have one already.
Creative Cloud Extract is a free new feature in Creative Cloud Assets
that helps you explore a PSD directly in your browser, including
layers and layer compositions. It particularly benefits web designers
and developers who can share PSDs, unlock design information, and
download production-ready assets. Using Extract, you can perform the
following actions with a PSD file in the browser:
- Copy text and CSS
- Get color, gradient, and font information
- Measure distances between elements
- Save optimized image assets for production
I think this is the most reliable way to view PSDs outside of using one of Adobe's paid products. As others mentioned, third-party programs might not be able to properly open PSD files. Since it is an official Adobe product and since Adobe created Extract with developers in mind, I can be reasonably sure that it will be fully compatible with any PSD created from Photoshop.
An even better option is Brackets, a open source code editor. The latest version actually comes with a preview version of Extract that is built into the editor. One of the coolest features I discovered is that you can open a PSD and while typing CSS code, it will give you hints based on the currently selected layer. You can also select a layer with an image, or a group of layers, and it will give you the option to generate an image (from the selected layers) as soon as you are typing code where an image path is expected (for example the src attribute of an img element).