Why blog calendar widgets are bad
I've kind of written about this before, but real blog usability drives home why calendar widgets in blogs are a waste of space for most purposes: as a navigational element, they are at best very weak hooks to get people to explore the rest of your blog. There are two parts of this.
The first is that they give people very little or no information on what they'll get when they click on a link (or even when they look at it and think about whether or not to click). In a typical calendar layout the best you can do is give people the title or titles of entries if they hover over a particular day, and most blog calendar widgets don't bother doing even that.
The second is that navigating through a typical calendar interface is tedious and time consuming. Part of real blog usability is giving people something right away, because if they click a link and don't get something interesting on that page they're quite likely to stop and leave your blog. And if you have a calendar widget that gives people actual entries when they click on things, next look at how much more work it takes to navigate effectively with it, how much work it takes for a visitor to read a third new entry (both in terms of clicks and in terms of mental work to keep track of things).
Let's face it; if calendar widgets are really useful for anyone, it is for the blog's author. So the right place for them is in your administrative interface, not in the view that visitors see.
(I think that there is a role for 'archive' interfaces in general, although not necessarily a very big one, but a calendar widget is a terrible interface for that too.)