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; it also appears as the letterspace package
in some distributions).
The tracking package has similar facilities.
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.)