FORTUNE — One of the most reliable marketplaces on the web got its start as a humble email list. In 1995, Craig Newmark, a software engineer at Charles Schwab (SCHW), founded Craigslist as a way for friends and colleagues to keep tabs on San Francisco’s bustling arts and tech scene. But Craigslist soon became something bigger. It transmogrified into a virtual marketplace with listings for almost everything: jobs, apartment rentals, ride-sharing requests, antiques for sale, even casual sex. Indeed, before businesses like LinkedIn (LNKD), Airbnb, Uber, and Grindr populated desktops and smartphones, there was Craigslist.
Nearly 20 years later, little has changed. Its appearance remains utilitarian at best, a five-column website of blue on white resembling a holdover from the days of Geocities and Netscape. And it still only makes money by charging for a small percentage of ads, mainly job and real-estate-related. (The company has been profitable since at least 1999, according to a source familiar with the company.) Meanwhile, newer competitors are chomping at the opportunity to offer a sliver of the Craigslist experience, but better. Uber’s ride-sharing business, for instance, is booming thanks to a sharp mobile experience where users can track their driver’s ETA on a digital map; Grindr leverages the smartphone’s GPS chip to startling effect, surfacing the profiles of nearby gay men down to the last yard. In an industry based around the idea that a company must “innovate or die,” Craigslist has taken the opposite approach and persevered.
In fact, even the competition will credit Craigslist for being an Internet pioneer. “At the time that Craigslist launched, no question, it pioneered the way people found their local information,” says Kira Wampler, chief marketing officer of the real-estate startup Trulia. Says Joel Simkhai, CEO of the gay app Grindr: “I respect them a lot. They are a great startup for connecting people and creating communities that grew through word of mouth.”
MORE: Pebble sold 400,000 smartwatches last year, on track to double revenues in 2014
It is possible these newer services are taking somewhat of a toll on Craigslist. On its site, Craigslist reports 60 million monthly active users in the U.S. Alone, but that number could actually be closer to 34 million according to Quantcast, down from nearly 39 million in April 2013. (Craigslist CEO Jim Buckmaster declined to be interviewed for this story.) Either way, that’s an astonishingly healthy user base given Craigslist’s resistance to change.
That very philosophy may explain why Craigslist remains one of the top 50 web sites in the U.S, according to Alexa.com. Jeff Jarvis, a friend of Newmark’s and writer of the blog Buzzmachine, argues Craigslist’s simplicity is its strongest asset. “I think it’s a mistake to look at Craigslist as a site,” says Jarvis, who views it as a utility or a community of people. “In that case, the tool matters less. We in media keep thinking about the presentation and the thoughts on the screen. Craig thinks about the value that’s delivered, and I think that’s what matters more.” Indeed, Jarvis, a professor at the CUNY Graduate School of Journalism in New York, says Craigslist remains just as relevant today as it was a decade ago. Says Jarvis: “When I turn to my students in journalism school, I ask them where they got their apartments. Everyone raises their hand and says Craigslist.”
And while the business side of Craigslist may seem inert, it has made baby steps. In 2012, Craigslist sued the apartment listings site PadMapper, which draws a portion of PadMapper’s data culled from Craigslist. A judge dismissed charges of copyright infringement in May 2013, but Craigslist eventually responded by integrating maps into its own listings soon after.
MORE: Cisco’s Padmasree Warrior: Why I’m backing Box
Craigslist’s true achilles heel, if it has any, could be trust. Site visitors are allowed to post listings, but it’s virtually impossible for the everyday person to know their identity without the lister’s consent. “If you’re going to meet someone in person, the idea of being someone who’s been peer-reviewed could be an advantage,” explains Andrew Jones, an analyst for San Mateo, Calif.-based Altimeter Group. Simkai agrees: “Unlike Grindr, Craigslist users do not have a set, verified identity — they can change their identity for each post, which makes it best suited for one-off transactions — though it doesn’t necessarily make for a good, trusted dating platform.”
That may be the case, but pundits like Jones believe Craigslist will be around for another 20 years. Because whether newer upstarts like it or not, its community remains substantial, and at its most basic, it’s mastered what many other Valley businesses haven’t: It just works.