It depends. What story is your business telling, and to which audience(s)?

By default, I'd suggest one website, and enough discipline to tell a coherent story that drives an action (a sale, for instance).

If the business has distinct customers or visitors, where the story for one doesn't apply to the other, then separate sites might make good sense. Then you're essentially telling different stories to different groups.

There's just no one answer for every situation.

I will try to answer this question from a technical perspective as it has notable merits.

Having all the initiatives or different parts of the business under the same domain name or website is beneficial from your domain visibility and traffic standpoint. Even if different parts of the business are given a different domain name you can potentially send the traffic to your main website thus increasing the web traffic numbers.

Example

Google: Google gmail is easy to remember with a domain name "gmail.com" but the url gets transferred to mail.google.com. Similarly the Google's new phone nexus one is hosted at google.com/phone and voice service at google.com/voice

You can easily give each part of your business a unique and short name and keep it easy for your customers to remember and do the necessary magic to make these different sites (or micro sites) come under the main domain name.

Technical details

It is quite easy to manage the subdomains (e.g.: in a domain name http://forums.domainname.com/ forums is the domain name) with simple redirects and web server settings.

This helps your alexa ranking, google page rank, SEM, SEO and lot more.

A separate domain or site may be necessary when you do not want the users/subscribers to feel that you are sharing the information behind the scenes. Case in point is this forum inbound.org. Even though this forum is sponsored/supported by HubSpot they have this clearly separated it from their for-profit venture. Dont we all love HubSpot! Kudos to HubSpot team!

Hth

I vote for one website, with clearly defined sections. My company used to have several websites--main corporate one, a newsletter content based one, a custom job for a particular client to access the newsletter client, and a borrower login. It was really confusing and terrible to keep them all straight. We now have one big website with everything on it, and it has sections for the different consistencies that access it.

Also, on the subject of having different websites for one company--it can look spammy. I got a call from a headhunter a few weeks ago, and I googled the company and found about five iterations of the company's name flooding google, and it immediately screamed: "we're flooding google with spam so you won't find bad stuff written by outsiders on the web."

I agree with roderickm. “There's just no one answer for every situation.” It depends on your objective and target audience.

For reputation management you want multiple Web sites appearing in search engines to push the "bad stuff" off the first page of results. This can be accomplished by using different sub-domains for each section of a site, or with separate domains for a corporate site, brand marketing site(s), store site and/or blog. Add various social media sites including Twitter and Facebook and throw in some directory profiles including Wikipedia and the first page is filled with "good stuff".

For e-commerce purposes you should try to dominate the top three results. Again a separate corporate site, brand site and store site properly optimized can push your competitors and/or affiliates further down the results list.

If you have distinctively different audiences for products under the same brand then you need different sites for usability purposes. The look and feel of a consumer focused site may need to be different from the commercially targeted site and also from the investor site.

For globalization and localization you should consider using different country code top-level domains (ccTLD). If research finds that your customers prefer to buy local or if your product line is very different by country then distinctly different sites are warranted. If a searcher specifies "Pages in Mexico" then the ccTLD is the best way for the search engine to filter the results. Of course you can always use Webmaster Tools to tell Google that your dot com sub-directory is country specific but the results don’t look local to your customer.

So determine…

1. your objective
2. your target audience(s)
3. tactics that will move each target audience toward your objective

You may find that one site is enough, or not.

I'd always opt for one. The benefits all drive towards one result. You can also spin one website of a CMS if you plan it well. The rise of subdomains and microsites seems to rise from the 'closed' IT depts of larger organisations that cannot allow access to their main site and its historical position. So microsites have become a quick way of marketing a product in its own right. But you start cold rather than spinning off the back of a successful URL.

From purely a user perspective, consider this:

I've opened many tabs of links while searching. I flit between each of the sites while I'm waiting for loading, get a phone call, etc. What I thought I clicked on is not what appears on the page. Do I spend time hunting around your site when I have 6 other tabs open?

I'm very much interested in everyone's thoughts.