Pepsi Blue was a "Berry Cola Infusion" released in 2002. It's seen as one of Pepsi's biggest commercial failures because of the massive marketing push put into the product. The biggest problem with Pepsi Blue was that I personally expected it to taste kind of like Pepsi with berry, which it obviously didn't. It tasted like some kind of cleaning product instead.
2. Pepsi Twist
This was one of my personal favorites. It was first introduced in 2000. According to Wikipedia: The product appears to be no longer actively marketed and does not appear on the list of brands on Pepsi's official website. Pepsi Twist production was ended in the U.S. during the summer of 2006 however the drink is still available in Malaysia and some other nations.
Surge was first released in the US in 1996. It was discontinued in the USA, Denmark and Sweden in 2001 and Coke stopped making the syrup for fountains in 2002. Norway is the only country where you can still buy it.
4. Coke Blak
While I think this was a horrible idea, I'm really bummed I never got to try it. Coke Blak was introduced to the US as a coke/coffee combo in 2006. An eight-ounce bottle of Coca-Cola Blak contained 46 milligrams of caffeine. The drink was discontinued in 2008.
5. Dr. Pepper Red Fusion
According to Wikipedia: First introduced in 2002, it was available only in the US. The fruity, red-colored Red Fusion was the first new flavor added to the Dr Pepper family of beverages in the company's 122-year history. Its production was essentially canceled less than a year later, although in certain areas it was available until late 2004.
6. Mountain Dew Code Red
This stuff is GOOD. While it hasn't been completely discontinued, it's nearly impossible to find a 2-liter bottle in stores since Walmart and other stores stopped carrying it. The only place to find Code Red seems to be in vending machines. Read more about the shortage here: Why is Mountain Dew Code Red Disappearing from Store Shelves?
7. Black Cherry Vanilla Coke
The first year it came out (2002) Vanilla Coke saw its best sales. It's estimated that the product was purchased by about 29% of all United States households that year! But sales didn't stay so high... By 2004 sales were down to 35 million unit cases compared to 90 million in 2002. It was announced in 2005 that coke would begin phasing out Vanilla Coke in the US. BUT, luckily, thank God, Vanilla Coke was reintroduced in 2007. In January of 2007 Black Cherry Vanilla was released. Pretty good stuff. This one didn't make it, though.