I found a few screensavers that can use HTML files, but I don't have time to look into it until next week unfortunately, as I will be busy the rest of the week.
If anyone has time they could try to get it to work:
Disclaimer: Beware of viruses, I cannot guarantee that a quick google search will only give safe results. However as long as you have antivirus running it should probably be fine.
Also makes for a neat screensaver using HTML Screensaver using this auto play link
Download http://myweb.tiscali.co.uk/djmclean/htmlscreensaver.html, go to your computers screensaver settings and paste /u/mncke 's automatically generated up to date map of place https://abra.me/place-snaps/recent.png
I had the same problem with dual monitors, but the .scr I use lets you specify a different 'web page' to show on each monitor. I have the fakeupdate on one and a quick MS Paint file appropriately named black.png on the other.
From the comments on the stream, if you're on Windows install this:
http://myweb.tiscali.co.uk/djmclean/htmlscreensaver.html
Then point it at this:
http://www.ustream.tv/channel/iss-hdev-payload/pop-out
Didn't look into the Mac options, but someone mentioned it in the comment thread.
I use this one: http://myweb.tiscali.co.uk/djmclean/htmlscreensaver.html
I'm not using an RSS scraper right now, but you could use SimplePie or something. http://simplepie.org/
You'll need to set up a webpage (or static local page w/ javascript widgets) of what you want to show.
I made an html screen saver using --> http://myweb.tiscali.co.uk/djmclean/htmlscreensaver.html
Then used these 2 URLS for my 2 monitors:
Main monitor: http://www.ustream.tv/channel/iss-hdev-payload/pop-out
2nd monitor: http://www.lizard-tail.com/isana/tracking/
Take a look at this. It allows you to set HTML Pages as a screensaver.
Then you just need to embed reddit into an ifrace like:
<html><head></head><body><iframe src="https://reddit.com/r/EarthPorn"></iframe></body></html>
Then set the above HTML code as screensaver using the program et voila reddit as screensaver.
Depends what you mean by screensaver. If it's just a full-screen slideshow that you're after, then what migrego suggested below is enough. If you want it to start playing automatically after a period of idle time, then you can find a variety of freeware software that does this - display any web page you give it as a screen saver. e.g. some googling yielded this: http://myweb.tiscali.co.uk/djmclean/htmlscreensaver.html