You can't know everything. Hell, you could pick one language like Java and you couldn't know everything about that between all the libraries, APIs, technologies and platforms that come under the Java umbrella.
You can however be aware of (almost) everything. By "aware of" I mean you can desribe in 25 words of so what it is and what it relates to (eg "Swing is a library for writing GUI applications in Java").
How do you get to that point? Aggregation sites are you friends. I'll look at a few sites (almost daily) and scan the summaries. I might only read 1 article in 20 (or even 50). My usual list includes:
Occasionally some others including The Server Side (moreso in years past), JavaWorld, even StackOverflow (the questions that get asked give you a good idea of what's occupying developer mindshare at the moment).
I don't tend to listen to podcasts or read blogs religiously. I'll read them when they come up in searches or when they're reference on DZone, etc. I know others do (sometimes a lot).
Anyway, through skimming all this information (because reading it all would be 1 or even 2 full-time jobs) you should be aware of most technologies and, by the number of times they're mentioned, you get an idea of how they're trending and how large a mindshare they have. That's how I got into jQuery (it became very big last year).
It's also how you know ColdFusion is legacy: I don't think I saw a single article on it last year. Sure people use it and sure releases are still coming out but it is relegated to legacy now.
I used to read a lot of books. Not so much anymore. There's too much good information on the internet already. Some books warrant the purchase (eg Java Concurrency in Practice by Brian Goetz et al).
Forget magazines. They're an anachronism.
So there are some things I can do. A few of them I can (hopefully) do very well. But there's an awful lot of other things I can't do but know about, know some pros and cons and can have a meaningful (high-level) conversation about. And I think that's where you want to be as a developer.