Wow, lots of good advice given already.
I mostly shoot portraits and tabletop in a studio with hard key and some level of fill.
The sun makes a remarkably hard key, having an angular size of about ½ degree (close to a point source). The frosted cover glass of a bare Profoto head is a great approximation of the sun but, being 7cm in diameter, must be placed 8m away from the subject to achieve the same angular size. At this distance, the lighting efficiency is unusably low. One workable cheat is bringing the head within 2m. A similar cheat is a Profoto MultiSpot (with 7cm frosted fresnel) at 2m for greater lighting efficiency. The shadows are softer-edged, but still plausible. For realistic sun emulation, I break out the ProZoomSpot which is a theatrical followspot that fully collimates the beam: it's huge, and overkill for most cases.
Fill is the other daylight element, and can suggest time of day and location by way of the fill ratio and color cast. I don't like blue, but I've seen Cindy Sherman work blue shadows into her studio creations. Green cast might suggest a forest. Yellow might suggest sunset (with the right key angle), or autumn (perhaps with a cucoloris). It's hard to suggest a portrait in midday sun unless the subject reclines, or you accept raccoon eyes.
Truest sunlight emulation with ProZoomSpot, no fill:
http://patternassociates.com/rico/nikon/misc/d300d3000.jpg
Early morning light from the side w/MultiSpot fresnel, fill card:
http://patternassociates.com/rico/leica/misc/lilac2.jpg
Early morning light ahead of camera w/MultiSpot, shoot-through panel for fill:
http://patternassociates.com/rico/nikon/misc/sandals.jpg
Midday light w/MultiSpot, same fill:
http://patternassociates.com/rico/nikon/misc/sandals4.jpg
Late afternoon light w/MultiSpot, medium softbox fill:
http://patternassociates.com/rico/photo/misc/jade19.jpg
Even though AM and PM share the same key angles, I always think of morning light as cleaner and thinner, while afternoon lighter is yellower and hazier.
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Canon, Nikon, Contax RTS, Leica M, Profoto