I don't know how anyone measured it, but someone promoted the claim that William S. Burroughs had the largest working vocabulary of... well, I don't even know what the standard was. Of everyone? What a proud claim, and only the most undereducated of his most fanatic followers would suggest such a thing. Certainly, we could say he was among the most literate of his peers or of his contemporaries.
I don't bring this up to denigrate him, far from it: I think he's an icon of aspiration in this sense. It's a fun game to collect as many obscure words as possible, but it's also essential to remain abreast of far-flung vocabulary just to keep one's mind in prime shape. And who's to say which obscure word won't be on everyone's tongue tomorrow morning, or what once-handy term will next find itself camping out in the outskirts of popular culture?
Otherwise, many online dictionaries or special-interest groups also offer WotD (word of the day) functions, and I've developed some preferences in using them.
Wordsmith.org - Anu Garg's WotD project has been around almost as long as the Internet has, and probably before. He's dedicated to exploring the world of English language and reflecting it back to its native speakers. It's an excellent one for beginners, replete with etymology and usage, but an advanced wordmonger may become frustrated at not discovering anything new by this service.
Merriam-Webster - Also set up for beginners, this is a facile and accessible list of words that you may encounter often or rarely, depending on how much you read for recreation. And if you grew up playing Dungeons & Dragons, you probably have a better vocabulary than anyone discovering a love of literature via Twilight and Harry Potter. No disrespect intended to those very popular works; I'm just saying AD&D boosted my working vocabulary in surprising ways. The words M-W's WotD produces are useful and sometimes interesting, but not abstruse enough for my preference.
Dictionary.com - I would warn people away from this website simply because it tends to dump 100+ cookies and data trackers onto your poor computer, but if you're a diligent computer user with antivirus programs better than McAfee and Norton and you're using Firefox or Chrome for a browser, you can go here long enough to grab the RSS feed. The words it produces are very good: I'd rate them between uncommon and obscure. If it weren't so data-invasive, I'd freely recommend this site to anyone ready for a taste of advanced vocabulary.
Oxford English Dictionary - The OED's online service is pay-only: you can't access this lovely database of knowledge without shelling out a pretty penny for it. I have access to it through my alma mater and I do cherish it. But! They offer a free WotD program! How about that? Mind you, Merriam-Webster is our nation's Declaration of Independence from the OED, but if you really want to study, really want to know where words come from, the OED is invaluable. A random selection of words from this source in particular is generally a delight.
Wordnik - Why do I like this one best? Is it the design/presentation? Is it the lines of classical text used for their case-in-point examples? Is it the truly, truly obscure selection of words from all up and down the timeline? All of these things, I'm sure, and more are why I value Wordnik so highly.
Best of all is grabbing the RSS feeds for each of these sites and compiling them wherever you like, whatever RSS-feed aggregate you like, embedding them on your own blog, whatever. The important thing is just to keep practicing new words, always, always, always.
* No, that's not a real word. I made it up, but you can discern what it means, right? It sounds real, doesn't it?