A common technique in advertising copy (and other text whose actual content need not actually be read) is to alter the space between the letters (otherwise known as the tracking). As a general rule, this is a very bad idea: it detracts from legibility, which is contrary to the principles of typesetting (any respectable font you might be using should already have optimum tracking built into it).
The great type designer, Eric Gill, is credited with saying “he who would letterspace lower-case text, would steal sheep”. (The attribution is probably apocryphal: others are also credited with the remark. Stealing sheep was, in the 19th century, a capital offence in Britain.) As the remark suggests, though, letterspacing of upper-case text is less awful a crime; the technique used also to be used for emphasis of text set in Fraktur (or similar) fonts.
Straightforward macros (usable, in principle, with any TeX macro
package) may be found in letterspacing (which is the name of
the tex file).
A more comprehensive solution is to be found in the soul package (which is optimised for use with LaTeX, but also works with Plain TeX). Soul also permits hyphenation of letterspaced text; Gill’s view of such an activity is not (even apocryphally) recorded. (Spacing-out forms part of the name of soul; the other half is described in another question.)
Possibly the ‘ultimate’ in this field is the microtype,
which uses the micro-typography capabilities of current PDFTeX to
provide a \textls command, which operates according to parameters
declared in a \SetTracking command. Microtype’s
‘tracking’ facility expands the natural spacing of the font itself,
rather than inserting space between characters. Ordinarily,
letter-spacing will destroy ligatures; however, this is wrong
for some font styles (for example, fraktur), and the
package provides a means of protecting the ligatures in a
letter-spaced text.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=letterspace