I've been using Wikis for many years now - the VW Wiki at UIUC has been a valuable resource for Smalltalk developers for a long time, and the Cincom Smalltalk Wiki is a place where we (the Cincom Smalltalk team) put out information on what we have coming down the pipeline. As a team, we use Wikis for internal communication as well. Periodically, I get asked "So what's the difference between a Wiki and a blog? Both allow for user editable web page - why use one over the other?"
It's a good question. As it happens, the two kinds of websites have some rough similarities (both are user editable in some sense) - but they also differ quite a bit. In broad terms, a Wiki has what you might call an emergent voice - it's an agglomeration of the input of the community of users who are interested in the content being managed. Here's an example - the history for one of the pages on the VW Wiki:
See how there's a long list of editing changes across time? That page was originally created in October of 1999 - and the last (real) edit of it was in April of this year. Looking at the list of edits, you can see that the page was edited by more than one person across a long time interval. As the interests of people have waxed and waned, content has been added (or subtracted) from the page. Over time, the direction of a page tends to focus in - at least on this wiki.
Other wikis can come to resemble conversations - have a look at the popular C2 Wiki, originally started by Ward Cunnigham many years ago. A glance at the recent changes page shows many changes over the course of a single day - have a look at this page, for instance - you can see that many people have contributed content over a long period of time.
A popular wiki can quickly spawn off dozens of side points and conversations - a good example of this is the Atom Wiki, created to engender conversation and conclusions on the Atom syndication format (an alternative to RSS). The limitations of a wiki for conversational purposes cropped up rather quickly here, and spawned a mailing list. That list tends to have dozens (sometimes over 100) messages a day, quite frequently on the finer points of things like date formats. This is barely manageable as a mailing list; it completely fell apart as a Wiki
In general, Wikis work well when you have a fairly limited number of people contributing content, and a larger body of readers and occasional contributors. If the amount of content creation spirals too quickly, there's just too much information - it becomes a lot like a USENET group that's been taken over by trolls. Even if most of the content is good, there is such a thing as too much of a good thing.
How do blogs play into this? Blogs allow for a much tighter focus on editorial control. You can disable comments, for instance - and thus ensure that only one voice is being heard. Even with comments enabled, they typically are not shown when a visitor comes to the site - the visitor has to choose to view comments (as opposed to a Wiki, where anyone can add any content they want anywhere on a page).
Have a look at this screenshot of my blog, for instance:
There are individual articles, and the start/end of an article is determined by the author. With a wiki, a reader can re-edit to their heart's content; a blog is much more like an editorial page - and comments are akin to letters to the editor. The author of the blog retains ultimate control; he can choose to allow comments or not, and - like a newspaper - decide whether a given comment (or article) stays or goes.
With a wiki, you don't have full control over the content - and attempting to keep control will lead to "last one to edit wins" types of battles. A wiki is most useful when you want to create a community accessible site, where constructive criticism is encouraged. A blog is far more useful for getting your specific message out, without changes being made by someone else. Sure, other bloggers can make comparison posts linking to yours (I do that all the time) - but the author still retains complete control over the content - and ultimately, over the message.
There's another very large difference as well. A wiki is much more like a "traditional" website than a blog is. Once a page is created on a Wiki, it's just there (unless, it's deleted, just as with any other site). A blog is much more ephemeral. Typically, the main blog page will show the last N items or the last N days worth of items. Sure, everything is accessible via the archives or via search - but it's not just there. The top page of a Wiki, with the items it points to, is a (relative) constant. The top page of a blog changes with every posting. Thus, a Wiki is better for information that needs to be "sticky" - a blog is much more akin to the morning newspaper. Fresh, but changing.
So, getting back to the comparison - under which circumstances do you want a wiki, and under which do you want a blog? If you want to encourage input from a community, a wiki is a good tool for that - however, you need to be aware of the limitations. Most people are uncomfortable writing html, and many people will be uncomfortable using wiki markup (most wikis support a simplified markup scheme). Given that, you are going to get feedback from a limited subset of the total readership. This subset will be even smaller if you aren't targeting a technical audience. If you want to get a personal voice "out there" talking about your products and services, blogs are the way to go. A caveat here though - a blog is only useful if the author(s) post regularly on topics of interest to the target audience. It's very easy to start a blog - it's much more difficult to sustain one.
In many cases, you are going to want to use wikis for some circumstances, and blogs for others. Either way, commitment is very important. Blogs need regular postings - wikis need maintenance and pruning. If you plan to use either or both, you need to walk into the venture knowing full well that a decent sized time commitment is necessary.