Internet portals like Prodigy, CompuServe, iMagination, etc. They were called portals because that's how you usually entered the Internet — they had a lot of links to useful sites, news and a content listing. When the Internet was fledgling during the '90s, they were extremely popular. However, the more efficient, less resource-intensive, and free World Wide Web put them on a steady decline. Today, America Online is the only one of these services that still remains, and even that's pretty much on its last legs, remembered as a symbol of all that was wrong with the mainstream internet in the '90s.
Related to the above, dedicated instant messenger programs (such as ICQ, AOL Instant Messenger, and MSN Messenger) for computers were very popular among internet users during the days when a significant percentage of them had dial-up connections, as they would allow you to see when a friend logged on and instantly open a chat with them. But the rise in popularity of text messaging, cell phone plans with free long distance calling, social media sites like Facebook adding built-in chat features, gaming clients like Steam and Battle.Net having built-in instant messaging, and free video chat programs like Skype have caused dedicated instant messengers to become somewhat obsolete. These days, IM programs are mainly used by businesses as a way of allowing quick, easy communications between employees who may not be sitting right next to each other.
Having certain e-mail providers are seen as sure signs that you are an old person who probably barely uses the internet. These include Yahoo, Hotmail, AOL, and various discontinued internet providers like Netzero. In fact, pretty much any e-mail provider other than Gmail seems to be slipping into this trope.
However, Hotmail may end up seeing a second life as Outlook (since the Hotmail accounts were grandfathered when the change occurred).
The dot-com boom in the late '90s produced a great many websites and companies that were built seemingly entirely on hype in business journals and kitschy adverts, among them Pets.com, Lycos, and Freeinternet.com. After the bubble burst in 2000-2001 and dragged down many e-businesses with it, they went from being the subject of public admiration for their founders' ingenuity and entrepreneurial spirit to mockery for their folly virtually overnight. This E-Trade Super Bowl ad from 2001 (a parody of the Crying Indian ad) demonstrates the fall from grace that many such sites underwent.
"Tribute" videos on YouTube. Back in the heyday of 2006-2009, it was common among the younger users to make slideshows/video collages of something, usually a sports team or fictional character, set to a song (usually Linkin Park, Green Day, Simple Plan, Three Days Grace or similar). Nowadays, with platforms such as Tumblr where people can blog and discuss such things more extensively, such videos are now considered obsolete or redundant. It didn't help that the videos always had a Hatedom among the more "mature" users, who found them Narmy and cliche. An easy way of telling if a YouTube user was under the age of 17 was by whether or not they had a Sasuke tribute set to Animal I Have Become in their uploads.
Webrings, Topsites and internet directories seem to have came under this trope recently, with the increasingly prominence of Google (which most discounts their effects on a site's SEO) and the rise in social networks as a means of promotion having made things like giant lists of links and the like kind of useless. Indeed, some might say the only people who even visit such sites nowadays are webmasters trying to promote their work (instead of end users trying to find new content). This can be seen by how the Open Directory Project/DMOZ has slowly fell in relevance, and even completely been dropped as a feature by Google in recent years.
"Mature humor" Flash sites - like Newgrounds, Camp Chaos and Icebox - were seen as the next big thing in the late '90s. At the time, seeing something so independent and uncensored was absolutely phenomenal, especially to younger teenagers. Once the millennium hit though, the popularity of these sites took a downhill slope. Some of them got swept up in the economic dot com bubble burst and shut down, others tried avenues into other forms of media and didn't have much success. As the scope of the internet grew as well, some people just found the humor too juvenile to enjoy anymore. Newgrounds is still one of the few sites from then still doing well, and even they have tried to shy away from the tasteless humor aspect, instead mainly showcasing more art-oriented submissions.