The simplest way to create a divider line (in the html post or page editor, or in a text widget) is the hr tag:
This tag no longer supports attributes for color, thickness etc., so you cannot change the way the line looks. You can only make it shorter, by turning it into a constricted-width section, for example:
For a customizable line in a text widget, you can use a no-content section with specific dimensions and background color:
(For hex color codes see “Colors” under Links 1 in my sidebar.)
In the post or page editor you need this variant:
(The hyphen has been added because completely empty sections are stripped out if you switch the editor to visual; zero font size ensures it’s invisible.)
For a dotted or a dashed line, you can use a similar section with a colored top or bottom border (see relevant post) instead of background color, for example:
Same thing for a (continuous) double line. For a dotted or dashed double line, use colored top and bottom borders, for example:
• add margin-top:NUMBERpx;margin-bottom:NUMBERpx; to the array of commands if you need more space above and below the divider (to decrease the space, use negative numbers);
• use a percentage instead of px if you prefer proportional width;
• try align="center" if class="aligncenter" doesn’t work in the theme you’re using;
• remove the commands for alignment and width if you want a full-width divider.
Alternative for a full-width divider line: add a bottom border to the paragraph preceding the point of insertion or a top border to the paragraph that follows it; for example:
For fancier dividers you can google for “divider lines” (or “line dividers”) clip art.
Some specimens here: http://www.webweaver.nu/clipart/bar2.shtml.
Some other collections here: http://www.webplaces.com/html/lines.htm