Labor Day is well behind us, but it's not too late to go back to school. The proliferation of free online classes means that if you have the time and the curiosity, there's no reason to be ignorant about anything anymore.
The first massive online open courses (MOOCs) I signed up for were Peter Norvig and Sebastian Thrun's AI course and Andrew Ng's machine learning class, offered by Stanford in the fall of 2011. I joined because I'd heard about them on a news story on NPR. NPR was reporting that signups were exceeding all expectations, into the tens of thousands of students (the AI course ultimately had about 160,000). I figured that this was an experiment unlikely to be repeated and I'd better make sure I took advantage of it. Both courses were a lot of fun, and I found myself more motivated to learn and complete the course than if I'd just read books on those topics.
I've taken several more courses since then, both from Coursera and from Udacity, the two companies created after the successes of the Stanford courses in the fall of 2011 (Professor Ng cofounded Coursera, and and Professor Thrun Udacity). None of the classes has been graded quite as rigorously as a typical university undergraduate course, mainly because the constraints of automatic grading (required for classes with thousands of students) mean that a lot of test questions are multiple-choice. That said, the MOOCs have still been a very satisfying experience—I've learned plenty and, I think, retained as much as in a typical in-person class.
Right now I'm taking four Coursera courses: Tim Roughgarden's Algorithms Part 2 (I somehow managed to graduate without taking an algorithms class, but it's never too late); Martin Odersky's Functional Programming in Scala (so far a bit elementary, but everyone I've talked with who uses Scala seems to think it's worthwhile); Michael Geneserth's General Game Playing (I've always wondered how that works); and Dan Grossman's Programming Languages (an excuse to learn ML, Racket, and Ruby, none of which I know).
I still don't understand how so many MOOCs are being offered at absolutely no cost, and maybe it's a business model that won't last. Get 'em while they're hot! (I'm not sure what the Coursera signup deadlines are like, but the General Game Playing and Programming Languages classes have just begun, if you want to join me.)
And finally, if there's something you want to study for which no one has set up a MOOC, the modern world offers you another study alternative besides curling up in a corner with a book. I mentioned previously that the Scala Study Meetup Group is working its way through a textbook; the Bay Area Categories and Types group has been doing the same with Benjamin Pierce's Basic Category Theory for Computer Scientists (I still can't claim to understand much of category theory despite attending the meetups faithfully, but it's a start).
So if there's something you want to learn, don't sit around bemoaning your ignorance. Join a class or start a meetup group, have some fun, and find out just how much more there is to be ignorant about.