3D file viewer
- GitHub has a viewer now - 
- STL Viewer for Linux, works well in Ubuntu - Meshlab
- Quickparts adds 3D STL viewer to online quoting
- Free stl viewer helps in making your model 3D print ready - Their STL viewer detects whether a model is watertight, that is, all edges of the triangles of which the model consists of need to have a mutual edge with another triangle.
- FreeCAD opens STL files
- VariCAD Viewer is available for Linux
For embedding 3D models in web pages, it is suggested to export them in one of the following formats:
- X3DOM, which needs a modern, WebGL-enabled browser
- X3D (needs a browser plugin or Java Applet)
- VRML (needs a browser plugin or Java Applet)
- COLLADA (see the COLLADA file converter, which allows you to import files created using Google SketchUp, Autodesk 3ds Max, and Autodesk Maya into the WebGL implementation of O3D)
VRML is the Virtual Reality Modeling Language, a standard file format to display 3D models on the web. The most common way to display, show, navigate, browse, play, or view VRML files, models, or worlds (.wrl file extension) is with a VRML plugin, also known as a VRML viewer, player, browser, reader, add-on, client, toolkit, program, software, or ActiveX control. You must download and install the VRML plugin in Internet Explorer, Firefox, Google Chrome, or other browser. Most of the VRML plugins can also display X3D files. The current version of VRML is referred to as VRML 2.0 or VRML97.
X3D is an open standards file format and run-time architecture to represent and communicate 3D scenes and objects using XML. It can be thought of as the next generation of VRML. X3D files can be displayed with an X3D plugin, viewer, browser, program, or toolkit. Most of the X3D plugins also display VRML files.
X3DOM is a framework to support X3D in HTML and rendered with WebGL. No plugin is required although it is or will be supported in more recent versions of Firefox, Chrome, and Safari. 
For WebGL-Enabled Browsers
If you're using a WebGL-enabled browser, you can choose one of these options:
Blender to WebGL exporter
GLGE and COLLADA files
- Export the model to a COLLADA file
- Use GLGE to display it (TODO: explain how to do this)