Zoom Lenses
Canon zoom lenses range from ultra-wide angle to telephoto lenses. The lower the focal length (mm), the wider the angle. The higher the focal length, the further the zoom. The EOS cameras apply a 1.6x factor to all lens focal lengths. That means that a lens that has a focal length of 10mm on a EOS DSLR is actually a 16mm (10mm x 1.6). A focal length of 200mm is actually 320mm (200mm x 1.6). The great thing about lenses is that they retain their value extremely well. A lens that was purchased 2-3 years ago in good condition could sell for almost the same price it was purchased at. So it pays off to take care of your lens investments. Most would agree that quality lenses are more critical for producing great photos than the camera body itself. Not to mention they are interchangeable and can be reused every time you replace your camera body with the newest model.
See the Canon Focal Length Comparison to gain a better understanding of what results each prime lens will get you.
Depending on what type of photography you enjoy from landscape (wide angle) to sports (telephoto) or macro, you will want to get different lenses to cover the range you're looking for. Here's my gear list if you're interested in seeing why I chose the lenses I did.Here are some useful resources for lenses:
The following are lenses that are very popular and highly recommended for SLR uses of any experience level:
Canon EF-S 10-22mm f/3.5-4.5 USM
The 10-22 EF-S lens is the widest
ultra-wide angle Canon zoom lens for the EOS cameras. It can
get you some really cool shots of close ups as well as landscapes. It averages around
$800.
Canon EF 16-35mm f/2.8L USM
One of the
all-time favorites of Canon photographers, this L-series lens is a great wide-angle lens that some
photographers because of it's range and fast aperture (great for low-light situations). But
this lens isn't cheap. It averages for about $1400.
Canon EF 17-40mm f/4L USM
The 17-40 although not as fast (1 f-stop slower) as the 16-35, is still a great lens with similar range. The best part is that it's a more "affordable" L-series lens with an average price of about $700.
Canon EF-S 17-55mm f/2,8 IS USM
This lens has quickly become the walkaround lens of choice due to it's versatile range and super fast aperture of 2.8 with Image
Stabilization. You pay for what you get as this lens generally will run you over $1000. That being said the results are stunning and
despite this being an EF-S lens (cannot be used for full-frame cameras and prone to gathering dust under the lens), this lens stands up
against the L-series professional lenses. 
Canon EF-S 17-85mm f/4-5.6 IS USM
A popular replacement for the EOS kit lens is the 17-85 IS. It has the wide
end as well as a decent zoom. The IS (image stabilizer) is often considered equivalent to
one f-stop, although most pro photographers wouldn't agree. This affordable lens runs for
$600. Some would say that at this price range the 17-40 f/4 is more the way to
go.
Canon EF 24-70mm f/2.8L USM
The 24-70 f/2.8 is the most popular lens for wedding photographers.
It's fast aperture is good for action shots and low lights. The focal length range is
perfect for group shots and portraits. It's a bit on the heavier side not to mention a bit
pricey at $1100 or so.
Canon EF 24-105mm f/4L IS USM
The newest addition to the L-series family, the versatile and high performance EF 24-105mm f/4 has both great range and a
fixed f/4 aperture in a light and compact package. All that good stuff comes at a price of around $1,200.
Canon EF 28-135mm f/3.5-5.6 IS USM
The 28-135 is a great walk-around lens. The glass isn't the best since it's not an L-series lens but it has versatile range reaching up to 135mm (216mm effective length) and is very affordable lens at around $400. A lot of new DSLR owners get this camera as their first lens after the kit lens.
Canon EF 70-200mm f/4L USM
This was my first telephoto zoom lens and I love it. It's light, compact and takes amazing pictures. And it's a steal at only around $600 for L-series quality. Not the greatest in low-light, but outdoors it's hard to beat.
Canon EF 70-200mm f/2.8L USM
The f/2.8 without IS is a step up from the f/4 version of the 70-200 zoom. That also means its a step up in price and weight, but performs better in low light. But the extra stop is going to cost you almost twice as much as the f/4 at $1100.
Canon EF 70-200mm f/2.8L IS USM
Easily the fan-favorite big daddy of all Canon zoom lenses, the 70-200 f/2.8L IS USM is the telephoto zoom of choice with amazing colors and sharpness at both ends, this lens is great for sports, performances and more. This lens is a must have for most serious Canon photographers and runs a pretty penny at $1600-1700 a pop.
Canon EF 70-300mm f/4-5.6 IS USM
Added to the Canon family of EF lenses in 2005, this new 70-300 comes with IS and performs far better than its predecessor the 75-300 IS. This lens is compact, light, great range and a great value for under $600.
Canon EF 70-300mm f/4.5-5.6 DO IS USM
The 70-300 uses a new technology called diffractive optics that allows the telephoto lens to be more compact and lighter. Unfortunately, by doing so there is also a compromise in photo quality. This lens costs about $1300.
Canon EF 75-300mm f/4-5.6 III USM
Compact and lightweight telephoto zoom. Great for general purpose but the quality might leave you wanting more.
Tamron SP 28-75mm f/2.8 XR Di LD-IF
One of the best and most critically acclaimed third-party lens options out there and for an outstanding price of less than $350.