straight line is the most basic geometric shape, although it's rarely the first thing that comes to an average designer's mind.  The reason is simple---it's rarely useful: Unless you're drawing flow charts or site maps like this one, you usually don't need straight lines in any role other than boundaries of page elements (most often represented by rectangles).

One exception is the horizontal rule, a text punctuation tool which is so common that it's given an HTML tag of its own, the HR tag.  While useful on pages with lots of text, it isn't a particularly elegant device from a design viewpoint.  In many cases the urge to put an HR is a sign that either the contents or the layout of the page are not thought out very well.  The boundaries between page sections should be expressed by purely design means (layout, different background colors or fonts, etc.), and if these means seem insufficient, you may consider dividing the content into two or more pages.

It is also unfortunate that the default look of an HR is in most browsers bevelled (pseudo-3D) because this is almost sure to clash with the page's graphic elements (unless you have none, of course).  Indeed, these bevelled rules imply a very special "engraving" effect which may only distract when combined with body text that is absolutely flat and a heading that uses, say, drop shadows.  The HR is only appropriate for pages with minimum graphics or for some special cases where the designer knowingly seeks the engraved effect.

So please try to design your custom graphic rules to match your site's style or, at the very least, add the NOSHADE attribute to the HR tag---such a rule is more likely to look satisfactorily.  (But never, never use those motley animated parrot-like rules from "web art" collections!  If you're considering putting one on your page, then, oh well, you'd better stay with the good old HR.)

Getting back to straight lines in general, we discover that they are quite a popular design theme these days.  Thin lines of a prominent color (often black or white) can help integrate a composition of blurred shapeless graphics and pseudo-randomly laid out text blocks.  These lines connect, embrace, and relate the parts of an image; they're the natural "power lines" that our eyes tend to follow.  They undoubtedly originate from the lines on flowcharts and callouts; that's why they prefer horizontal and vertical directions, sometimes are dotted, and sometimes have arrowheads, notches, or bullets at their ends.