I mentioned in a comment recently the importance of using quality lenses to achieve the best results in digital photography (and analogue photography too if it comes to that). I don't intend going into vast detail here about what to look for when buying lenses for your DSLR, but thought instead I'd make a recommendation for an all-around general purpose workhorse.
Let me start by saying I'm a lover of prime lenses. Prime lenses are fixed at one focal length and do not zoom, so are designed to do a specific job. By comparison primes are more expensive to buy than zoom lenses if only because you have to buy a range to achieve the same focal lengths. The upside of primes is their superior image quality.
When I go out walking about in the countryside I like to have a wide angle lens (20mm), a 'standard' lens (50mm), and a short telephoto (85mm) in my bag. I usually carry a 70-200mm zoom too in case I need to 'get in close'. The weight of my camera body and those four lenses can become quite noticeable after four or five miles and my camera bag is a dead weight on a 20 mile hike... I decided last year to cut down a little.
An excellent alternative to the 20mm wide angle and 50mm standard lenses is Sigma's 18-50mm f/2.8 EX DC lens and I acquired one of these some time ago to use as my 'walkabout' lens.
These days if I go walking up on the hills I tend to just fit this one piece of glass to my camera and leave everything else behind.
The Sigma 18-50mm f/2.8 lens is an autofocus lens optimized for APS-sized digital sensors that provides an extremely useful range with a constant f2.8 maximum aperture ensuring nice bright images.
Despite the lens being AF, I tend to use manual focus much of the time as I feel it gives you slightly more control.
Attractively finished in Sigma's matte-black 'Pro' surfacing, the lens offers ribbed and rubberised zoom and manual-focus rings, and is well-marked with easily legible focusing, zoom, and macro scales. Internal focusing with a non-rotating front element means the lens can't be focused manually in AF mode.
Sigma's guide price for this lens when first announced was in the £300 bracket, but don't be fooled by that; I paid considerably less for mine from a main dealer, so it pays to shop around.
One of the things that makes this lens special for me is the crispness of the images it produces.
The above image was taken close to the 50mm end of the range at f/4 and the size has been reduced to fir nicely on the web. If we crop into the image and look at the actual size of the picture (below), you start to get some idea of the quality produced.
Click both the above for larger versions.
So there you have it... a 'walkabout' lens available in all major DSLR fittings that offers crisp images and superb value for money. I wouldn't be without mine now. 
antlady69
Pro 
You prefer prime lenses? You're such a purist, honey!
Not that I know as much about photography as you, but I agree (nevertheless) with the importance of having quality lenses. And not "mega many" pixels as advertisers like you to believe.
Even small "toy" digital cameras can make surprisingly good photos if they have a good lense (Nikon, Canon).
Sigma indeed seems to be an excellent - and less expensive! - alternative to original lenses. Thanks for this informative post. xx