A list of rafting companies websites:

Statewide Colorado Rafting Outfitter
Whitewater Rafting Colorado| Rafting Colorado
http://www.royalgorgeraft.com/
Breckenridge Whitewater Rafting
http://www.lostpaddlerafting.com
Bill Dvorak's Kayak and Rafting Expeditions
List of Rapid Routes/Areas and rapid class:
Cache la Poudre River: Class II & III trips and Class III & IV trips
- Royal Gorge (Arkansas River) 4/5
- Big Horn Sheep Canyon/Parkdale 3
- Pine Creek 4/5
- The Numbers 4/5
- The Narrows 3/4
- Brown's Canyon 2/3/4
- Milk Run 1/2 (family, 1.5 hours)
- Blue River 3
- Colorado River 1/2
- Clear Creek 2/3/4/5
- Arkansas River 2/3/4/5
- Clear Creek 2/3/4: for families up to advanced rafting
Class definitions:
- Bathtub / Floating
- Young Kids / Water rolling
- Intermediate (Good adult/teen beginner)
- Advanced
- Experienced/Aggressive - expect heavy paddling and self survival swimming
- Niagara Falls - *death*
General Rafting Information:
You should select your rafting trip on location, your experience and ability. Persons with medical conditions should seek their doctors advice first. A waiver of liability will be required by the company before you can participate. The company may have age/weight restrictions for certain rapids, especially for the higher classes. Companies should be licensed by the State of Colorado and National Park Service to operate on the river. This increases the taxes and fees for the operators, but also required them to carry insurance and provide first-aid training to their boat guides.
Peak runoff is typically mid-June. It's difficult to pick the exact date and water flows. Booking closer to your rafting day will allow you to monitor the water levels better. The water is melted snow, thus it is very cold, especially early in the season. It warms up over the summer. Reservoirs are also filling over the summer so the flow may be lower earlier and then suddenly raise up late in the season when they release water. The snowpack levels near the end of winter will also determine the flow of water over the summer. Highest runoffs do not mean the best ride however.
Half-day, full-day and multi-day trips are available. Half-day is recommended for smaller kids that may get bored easily or if you are unsure if you will like it. Full-day is the most typical trip, and multi-day camping trips are for people who want a longer trip. Overnight trips can be tailor made to suit the level of difficulty and excitement you're looking for. Full-day trips generally include a lunch break with food provided (sandwiches typically) and sometimes dinner (you should ask what's included before you book).
The Cache la Poudre River has trips for all skills levels, from the family float of the leisurely Class II rapids to the teeth-clenched, soil your wetsuit Class IV trips. There are five different companies on the river: A-1 Wildwater, Mountain Whitewater Descents, Rapid Transit, A Wanderlust Adventure, and Rocky Mountain Adventures (run out of Estes Park). The "Poudre" as it is locally known it roughly 20 minutes out of Fort Collins, CO. The has etched it's own route through the Rocky Mountains and provides rafting trips with high rocky cliffs and hills of towering evergreens to soak in during your journey.
Browns Canyon rafting trips are incredibly scenic and full of super fun Class III whitewater. It's also one of the most popular locations to raft, thus it can be quite packed with boats at times during peak season (July). Brown's Canyon is a proposed federal wilderness area and is removed from roads and traffic. You'll feel like you're really getting away from things there. There are lots of other great places to raft too. The Bighorn Sheep raft trip area/river is highly recommended since it's less crowded and really pretty. Royal Gorge is for experts/experienced persons and is one of the scenic run in the United States. Here you can raft under the worlds highest single span suspension bridge. The Numbers is a Class III-V section north of Buena Vista that offers some of the best and most continuous rafting around! For the most advanced challenge on the Arkansas River, check out trips on the Class IV-V Pine Creek.You can combine The Numbers and Pine Creek into one full day trip. AVA Colorado Rafting is the company that runs the most trips through the Numbers and Pine Creek sections. Wilderness Aware Rafting is based at the beginning of Browns Canyon section and also runs trips on all section of the Arkansas River..
Clear Creek offers the closest rafting to Denver (about 30 minutes away!) and there are trips of all levels. You'll find family-friendly trips for first timers, intermediate trips for the more adventurous sorts, and advanced trips for experienced rafters or really fit and adventurous types. A few different companies offer Clear Creek rafting trips .
HINT:
QUESTIONS TO ASK: When you call to book a trip, ask whether the company includes free wetsuits in their trips and whether they wear helmets on all of their trips. These are services you should expect when shopping for a rafting trip. Just a safety note: helmets are not required by the state of Colorado on Class III sections, but a rock isn't any softer in Class III water than in more advanced Class IV and V! So you'll want to find a company that requires a helmet on Class III trips too. This signifies a better focus on safety. Companies that run trips on Clear Creek include Breckenridge Whitewater Rafting, Mile Hi Rafting, and AVA Colorado Rafting.
North Platte River is also recommended because it's less crowded, goes through a wilderness area and has the same level of rapids and pretty views, but it is much further North (which is why it's less crowded) than the others.
Wildernress Aware Rafting operates on the North Platte through Northgate Canyon.
The Gunnison Gorge is a spectacular section of whitewater in the same geologic formation as the Blac\k Canyon of the Gunnison National Park.
The Dolores River in Southwest Colorado offers classic desert wilderness.
First timers should not go on class 5 rapids unless they are in good shape and the adventurous type (and 16+ years old). Class III rapids are a good trip for beginner adults. If you want to give class V a shot your first time out, consider doing a full day trip which links Bighorn Sheep Canyon class III in the am, with the Royal Gorge class V in the afternoon.
Smaller sized boats that hold 4-5 people instead of 9-10 are available and you get more wet, and is a little bit more of a fun ride. Think Corporate jet that seats 10 vs. airliners that seat 100. Oar boats are for those that don't want to paddle. If you select a paddle boat, expect to work -- the guide will depend on you to power the boat appropriately.
What to Wear
Most companies will have wetsuits to rent (it may be included/required). It is not mandatory to use them for some companies, but you should consider them. You should wear a swimsuit and non-cotton shirt. Life vests will be required. A hat and sunglasses with string are recommended too. Do not wear jeans . Suntan/sunblock/sunscreen lotion is recommended (be sure it says waterproof) of SPF15 or higher. Bring a change of clothes to change into afterwards. Raingear is recommended as well.