A Subreddit is a custom-made subforum on Reddit, the community-focused content aggregator that recently surpassed Digg in both traffic and influence. In my opinion, the addition of subreddits back in 2008 is the feature that enabled Reddit to beat Digg.

Three years ago, the reaction was mixed. One social media maven (ugh) wrote, "It might just put Reddit more prominently on the social media map." Another blogger declared subreddits were a huge mistake: "Cross-posts and repeats become common place, thus increasing the homogeneity of content across the board. Likewise, the same couple of individuals are able to dictate content and control over the majority of Reddit."

Three years later, it's clear that subreddits are the reason why the site is the de facto breeding ground for viral content on the web.

Why are they so special? You can make a subreddit about anything and make it private or public. Subreddits allow you to follow very specific areas of interest. There's a subreddit for New York City where people talk about best places to find Pho or why the F train is down.

Here are the ten most popular subreddits, by subscribers:

Of course, these are very general interests. There are dozens of subreddits that go deeper than politics, for instance. There are GLBT activism subreddits, a Ron Paul subreddit and a subreddit for Balkan politics.

You can set up a subforum, not about videogames, but about a specific videogame, that enables a niche community to post and rate news and ephemera culled from hundreds of fan blogs and other gaming sites. Starcraft, TF2, World of Warcraft, Rock Band and Civilization all have their own subreddits.

This long list of subreddits is great for browsing (There are over 3,000), and this searchable database provides lots of fun statistics. If you have an account, you can add subreddits to your frontpage (a feed of the top stories from each of your selected subreddits).

If there isn't an existing subreddit for your interest, you can easily create one if you've got a Reddit account. I've lurked Reddit for a couple of years but only recently opened an account. My experience there has improved dramatically since. Highly recommended. It allows you to filter the news you receive so it's not all news about the brand of turtlenecks Steve Jobs wears.

Active subreddits develop their own in-jokes and memes. The members of the Trees subreddit, for marijuana enthusiasts, call each other "Ents," a reference to the tree-like Lord of the Rings creatures. They have their own comics and, as you might expect, engage in endless discussions about obscure aspects of the art of getting baaaaaaked, man. There are nearly 50,000 people who subscribe to the Trees subreddit!

One of the best new memes (recently covered by Urlesque) came from a subreddit: Dog Fort, which spawned a special subreddit for potential pics to use in Dog Fort, and a competing Cat Fort subreddit with its own picture subreddit.

Some of the more obscure ones I could find with a cursory search include highbrow (Evolutionary Psychology), middlebrow (What's for Dinner), and lowbrow (Axe Reddit and Statuegropers). New subreddits are created every day for the goofiest (and most serious) of subjects.