Glimpse  (which  stands  for GLobal IMPlicit SEarch) is an indexing
and  query  system  that  allows  you  to  search through all your
files very quickly.  For example, finding 296 lines containing
'whitehouse' in 8750 files  occupying 104MB  took  6 seconds on a
SUN Sparc 5.  Glimpse supports most of agrep's options (agrep is our
powerful version  of grep)  including  approximate matching (e.g.,
finding mis- spelled words), Boolean queries,  and  even  some  limited
forms of regular expressions.  It is used in the same way, except that
you don't have to specify file names.  So,  if you are looking for a
needle anywhere in your file system, all you have to do is say
glimpse  needle  and  all  lines containing needle will appear preceded
by the file name.

WWW: http://glimpse.cs.arizona.edu/