Radio Wallah is of course the seminal Japanese transistor radio site by collector and radio historian Alan Kastner. Many of the very earliest and rarest Japanese transistor radios can be found here, along with valuable historical and technical data not available anywhere else — this is certainly the most influential and authoritative transistor radio site on the Web.
Michael Jack's Transistor Radios is a Flickr site that must have at least 1000 transistor radios on it — and they're all beautiful, and many of them are rare — without question this is the largest collection of significant US and Japanese transistor radios to be found on the Web.
Eric Wrobbel's booklets have become an integral part our hobby over the years — they have added a great deal to how we think about transistor radios as collectibles and as objects of design and history.
Transistor Radio Design, a site by James Butters, focuses on the cabinet design elements found in early transistor radios, as emphasized by many beautiful high-resolution close-up photos. But more significant (at least for me) is that there is also a lot of valuable historical information and technical data on this site, much of which can't be found elsewhere.
The M31 Galaxy of Transistor Radios was my first transistor radio site, begun in 1998 — it's old and out of date in many ways, but it has a lot of radios on its pages that I no longer own for showing on this newer site, so I'm including it here as a link.
The Transistor Radio Directory — Aldo Andreani's comprehensive listing of transistor device and transistor radio pages and sites.
Andrew Wylie's Semiconductor Site — great historical info on early semiconductors produced throughout the world.
Childhood Radio — Ron Mansfield is currently in the process of building a large and informative site with lots of info on transistor radio repair and restoration.