Note: If you are not a Rapids fan this will have pretty much no interest for you. This is basically an advertisment for the three supporters groups that make up the Rapids most hardcore fanbase.
The thing that really defines any football team's fanbase is the ultras or supporters groups. For years now, MLS has been trying to duck away from the "Soccer moms and 8 year olds" meme that used to be the bulk of its fanbase seemlingly. Some teams took far less time than others to get with the program, with DC United fans being some of the loudest, rowdiest and most numerous supporters groups since the early 1996 days when they were still the toast of the town and MLS' first dynasty. Some teams like the Rapids are just now catching up.
Reading the Rapids wikipedia page you'll note that it says flat out that 'The Rapids have always been shaky when it comes to Ultras.' Groups have come and gone but last year was the first true year of real, true football style support with the opening of the supporters terraces. If you're a new fan, someone who's just decided to bandwagon the squad or even a long-time fan who wants to get more involved in supporting your boys - that last one was pretty much me last year - I urge you to join one of the Rapids three fantastic supporters groups.
Supporters group politics aside - we try and avoid talking about that sort of thing here - what the Rapids could use more than anything is another man/woman cheering from the terraces or a seated section for the team. The atmosphere improved immeasurably last year after the 'big 3' had finally gelled into their seperate ways. The three you have to choose from are below in alphabetical order because I'm lazy and people like patterns.
Bulldog Supporters Group - The 'baby' as it were among the three groups, the BSG - the members usually call themselves the Bobbies - is a group formed before last season based out of the famed British Bulldog pub in downtown Denver. They've quickly gained in membership, filling up the left side of the supporters terrace by years end last season. The Bobbies are known for their fantastic tailgates which have involved stunts as grand as BBQing entire pigs. They run very reasonably priced busses for fans to the game from the Bulldog before every match and from it afterward, often several hours before it starts in order to get plenty of tailgating - and alcohol - into the fans systems. The BSG also announced that they would be expanding to a seated section to accomadate growing membership and demand this season.
Class VI - A very old school group, Class VI was formed by fans who were tired of being stared at when they dared support their team loudly, resulting in a fantastic seated supporters section. This group of Rapids fans fill up section 108 at the Dick every game, with their own brand of tifo and the most noise out of any place in the stadium that isn't the supporters terrace. Calling it a seated section may be a misnomer since you will rarely see these guys not on their feet cheering for the team. Their motto is 'No Fan Alone' and they have access to fantastic ticket prices in their tight knit group to prove it. They run most of their drinking operations out of the Celtic Tavern in downtown Denver.
Pid Army - Back in the day, there were two small but fiery supporters groups at Dick's Sporting Goods Park, the North Corner Council and the Centennial Firm. When the Rapids front office finally gave in and gave the fans the terrace they had been requesting for years, the two older groups joined forces and gave themselves the moniker of Gary Smith's Pid Army. They are known for peppering the landscape with elaborate signage and tifo, especially the large wall behind them in the terraces. Some of the oldest and most fiery Rapids fans can be found in their ranks standing on the right side of the supporters terrace. Like the other two groups, they also hold fantastic tailgates outside of the stadium before games begin.
Whichever group you may choose, it's not important. What's important is being a Rapids fan and cheering for your team and I urge any fans who don't belong to one of these organizations to join up and keep the Rapids chanting going louder and louder for years to come.