[sigmasd14]

I recommend the Tamron SP 17mm f3.5. Its really sharp for a rectalinear wide angle lens, even wide open. It has great contrast, low CA, its fairly light and compact and because its an Adaptall mount lens you can easily convert it to practically any brand of camera simply by buying the relevant brand related Adaptall adapter for it so if you decide to change camera brand or have two or more different brand cameras you can use the same lens on every one, saving you a bundle!
The version I have also has several built in filters which can be selected by turning a ring a at the front of the lens, but they are'nt really much use for digital work so I keep it on the normal setting.
Its gives really nice neutral colours with no strange casts like you can get with some other wide angle lenses.
I paid £149 for my practically mint model, which was reduced from £189, from a camera shop, but look on ebay and you should be able to find one cheaper than that.