Starting with CS2008 telligent introduced the ContentFragementList as well as several out of the box widgets.  This is pretty much old news, but I thought it would be good to cover a very basic scenario...

User story:  An admin of a site wants the ability to add a news flash from time to time, that appears on the right sidebar of all pages on their site..

To accomplish this we will add a <CSControl:ContentFragmentList> to the master.Master in the left sidebar column (above the lcr content region)

Add the following markup to the master.Master just about the LCR <asp:ContentPlaceHolder> : (Update the CssClasses as appropriate for your sidebar)

<CSControl:ContentFragmentList runat="server" Property="SiteSideWidgets">
<ItemTemplate>
<CSControl:ContentFragment runat="server">
<LeaderTemplate>
<div class="CommonContentBox">
<CSControl:ContentFragmentData Property="FragmentHeader" runat="server" Tag="H4" CssClass="CommonContentBoxHeader" />
<div class="CommonContentBoxContent">
</LeaderTemplate>
<TrailerTemplate>
</div>
<CSControl:ContentFragmentData ResourceName="ViewMore" LinkTo="More" runat="server" Tag="Div" CssClass="CommonContentBoxFooter" />
</div>
</TrailerTemplate>
</CSControl:ContentFragment>
</ItemTemplate>
</CSControl:ContentFragmentList>

To make this new ContentFragmentList accessable for editing via the control panel; edit your theme.config file and add the following to your <dynamicConfiguration> section.

Assuming you are using something similar to the Hawaii theme, you will already have a <propertyGroup> called sidebar, so add the following <property> to it...

<property id="SiteSideWidgets" Text="Site Widgets" applicationTypes="Core" dataType="Custom" controlType="CommunityServer.Controls.ContentFragmentsCustomControl, CommunityServer.Controls">
<defaultValue>
</defaultValue>
</property>

We will leave the default template blank, so when the time comes, the administrator can go into "Site Administration" under the Control Panel, to the "Site Content" / "Site Theme" tab, and in this case, you should see a "Sidebar" tab in the theme editor, and on that page, see the widget box to configure your new site wide widget panel.

Now the site admin, can add a highighted post, or even a "Generic Content" widget, to type an HTML note, all users will see on all pages of the site.

You can find much more detail about these controls at Ben Tied's post on the subject.

http://getben.com/archive/2008/04/18/community-server-2008-how-to-support-widgets-in-a-custom-theme.aspx

Note: even though you have defined this ContentFragmentList in a site wide location, because there are several different theme areas (a blog theme, and hub theme) you will need to add the <property> to a <DynamicConfiguration> <PropertyGroup> for each of these where you want the content to be visible.  It is not currently possible to have a ContentFragmentList share its configuration between different themes.