Nikon D5000. What lens to get?

I'm looking to purchase another lens. I'm at a stump with trying to figure out what lens is good for what I do. I'm a beginnerin photography so I need a bit (lots) of help with figuring out lenses. I do a lot of still life photos, architecture, and urban city like photos. The lens I already have is the Sigma AF 18-200mm f/3.5-6.3 DC OS (Optical Stabilizer) Zoom Lens. I would appreciate any info I get. I also would like to know about the fisheye lens, I'm in love with so many photos taken with that lense.

While a 'fish eye' lens is nice, it's too specialized to be very practical. However, a super wide IS a better choice and one that gives corner to corner coverage, not just a circle like image most fish eye lenses give.
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Nikon DOES offer a interesting lens called a rectilinear fish eye. It's 16mm f/2.8. At full frame it gives a whopping 180' field of view from top corner one side to bottom corner on the other. On your camera it will deliver about 110' wide angle, which is still not bad.
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http://www.kenrockwell.com/...1628af.htm.
With any fish eye lens your going to get distortion. Don't think otherwise. And accept it.
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Another good or useful lens would be a 50mm prime of f/1.8 for them really lower light images or Nikon's 60mm f/2.8 macro.
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Ask yourself, what shots are you not getting, that you want often and what lens would help you get them? Once the answer is in your head, get that lens.

If you want a single lens solution, the AF-S 18-200 mm VR is a good choice.
Landscape? AF-S 10-24 mm
Fisheye? AF 10.5 mm
Macro? AF-S 60 mm f/2.8
Sports? AF-S 200-400 mm f/4
Portrait? AF-S 50 mm f/1.4

As you can see, the subjects you tend to shoot will dictate the lens(es) you will need.

Unless your interest in photography is just a flash in the pan, it is recommended that you buy Nikkor lenses. A good used Nikkor lens is better in most cases than any generic lenses.

The exception to that rule are the Tokina lenses. They are excellent generic lenses and are in most cases just as good as the matching OEM lenses.