I haven't spent a lot of time with these program, so take my comments below with a grain of salt.

• Mol* - new collaboration between authors of NGL and LiteMol (TypeScript),
• NGL - rich UI and plenty of features, used on RCSB website,
• 3Dmol - lightweight with well-documented API,
• JSmol - JMol translated to JavaScript. Works with and without WebGL. The most popular viewer on the web.
• Molmil - from PDBj. No dependencies.
• LiteMol - announced by PDBe in 2016 (TypeScript).

Also worth knowing about:

• GLmol - the first WebGL viewer (unless jolecule was the first one?), started in 2011, with experimental support for el. den. visualization. Many newer ones are based on it - iview, icn3d, partly 3Dmol,
• pv - lightweight viewer started in 2013, stylish cartoons, no support for el. den.
• Speck - specialized in ambient occlusion. Reads only XYZ format.
• xtal.js - aimed at crystallographers, UglyMol was started as its fork.

And if you don't mind Java applets:

• AstexViewer - good for viewing electron density,
• JMol - one of the most popular desktop viewers works also as an applet.

Javascript assets are usually served minified and gzipped, so we only compare the minified-gzipped versions.

Note that all sizes are small comparing with sizes of electron density maps.

^{* JSmol.min.nojq.js is only 42KB gzipped, but requires much bigger dynamically loaded resources.}