Thanks to @jeromerenard, REDbot now has command-line support:

 

RED now has a little 'feedback' widget on the right-hand side, courtesy of UserVoice. Please leave suggestions and ideas!

I've just pushed a few changes to RED that should fix issues when setting request headers on IE8, as well as make the display easier on the eyes when using that browser. Please tell me (here, or in an issue) if you still see problems.

Also (belatedly; this has been in for a while), RED now checks some IE-specific response headers, including X-UA-Compatible, X-XSS-Protection, and the pre-check and post-check Cache-Control directives.
Have fun.

RED now allows you to see the response body as it did, by clicking "show body":

...which will replace the diagnostic messages with this:

The markup is coloured thanks to Google's code prettifier . Also, note that clicking on a link will run RED against it, replacing the old "show links" function.

RED now lets you set common and not-so-common HTTP request headers:

This is especially useful when, for example, you want to check to see if a site treats a specific browser differently, according to the User-Agent header.

When adding headers, keep in mind that RED isn't just a simple HTTP client; it actively probes resources using multiple requests with headers like Accept-Encoding, If-Modified-Since and If-None-Match to see how it will behave. If you try to set any of these headers, RED will warn you.

RED also has changed how it shows you the details of a specific message or header; if you mouse over one, you'll see a bubble like this appear:

The fine details are still being tweaked, so please be patient if you see some bugs here (and, of course, feel free to report them!).

Lots more is coming down the pipeline, stay tuned.

The biggest thing that RED has lacked, as compared to the Cacheability Engine, was the ability to test not only one URL, but also all of the linked assets on a Web page (if that URL is HTML).
 
RED can do this now, using the 'check links' option under a page's headers (available when the response is HTML):

When you follow this link, RED will request every image, head and frame link on the page and display a table summarising the results;

The results are hopefully self-explanatory, but you can mouse over the headers for details of what the columns mean. The 'Problems' column shows the 'warning' and 'bad' level messages for each asset, and you can either refer to the bottom of the screen or mouse over the numbers for more details.
 
I'm still tweaking the display of the table; there's a lot of information, but I think it's already better than the CE. Feedback appreciated, as always.

RED already has a Twitter feed, but that's mostly for the truly geeky -- mostly, it gets a tweet every time code is committed to the repository.

Like I said, geeky.

I wanted to have a channel to announce new features, fixes and other interesting things in; this is it. Soon I'm looking to get it hooked up so the latest post shows up on RED's front page.

Enjoy!