Lots of good info above, and I see that my
CHEAP MACRO article has been cited a couple times. Macro options have been dealt with, so I'll talk about wide-angle options. The good news: some exist. The not-so-good news: they mostly ain't cheap.
The basic questions here are: 1) How wide is wide? 2) What you gonna do with it? and 3) How much you wanna spend?
1) How wide is wide? On our APS-C-sensor dSLRs, the 'normal' focal length (the diagonal of the sensor) is 28-30mm, depending on the exact sensor size. Roughly 16-25mm are 'wide' and shorter than 16mm is 'ultrawide'. Ultrawides with edge-stretching distortion are called Rectilinear; those with edge-curving distortion are Fisheyes.
Pentax makes a wizard fisheye zoom, the DA10-17, the lens that brought me to Pentax, priced under US$500 new. But it's specialized. Sigma makes 8-16mm and 10-20mm rectilinear zooms, priced above US$500, but there are QC issues. The Pentax DA12-24 is great but pricier. I chose the Tamron 10-24 (under US$500) for my rectilinear ultrawide. Other choices in the neighborhood are Pentax 14mm and 15mm primes, and the bargain slightly-fishy Zenitar 16/2.8 for around US$200.
For not-so-wide, there are the popular kit-zoom replacements at 16-45 and 16-50 (Pentax) and 17-50 (Tamron, Sigma), the excellent DA21/3.2 Ltd prime, and various AF and MF primes around 20-24mm. If you're looking to shoot distortion-free 'scapes, use a 28mm or 31mm lens. If a little edge distortion is OK with you, a good lens in the 20-24mm range is fine.
2) What you gonna do with it? Landscapes don't move around much except in 'quake zones, so fast AF lenses aren't needed. I prefer shooting land-sky-sea-town 'scapes with manual primes: 16-21-24-28mm, and my 21 ain't fast. If you're shooting people and other creatures moving around, or in lower light, then a faster lens is good. I use old manual-focus f/2 primes at 24-28-35mm, and the Zenitar 16/2.8 when I have time to compose images that exploit its slight fishiness.
3) How much you wanna spend? I've mentioned some price points above. Except with ultrawides, fairly low-cost manual primes are abundant. I'll argue that AF is less important with wide glass because their DOF is so thick. My M42-screwmount Tokina-made 21/3.8 (well under US$100) when stopped-down to f/11 and prefocused to 2m, has DOF from 1m to infinity -- it becomes a super-duper P&S lens. If your budget extends up to US$500, then the Tamron 17-50/2.8 is probably the best deal around. Pretty wide, pretty fast, very good optics.
Hope I haven't wandered too much. Cheers!