Following a link that recently came up on the golf mailing list, I ran across this problem, which I thought was ripe for golfing. I thought their input format was ugly, so instead of taking the words to be ignored on standard input, they are passed as command-line arguments (this also complicates the golfing a bit). An example of proper behavior is:

% perl kwic.pl a an the
an ostrich
a big bird
the an bird
^D
a BIG bird
a big BIRD
the an BIRD
an OSTRICH
%
[download]

My best so far is an 85:

$i{$x}=1while$x=pop;s/\w+/$i{$&}or$k{$&}.="$`\U$&\E$'"/egwhile<>;print
+@k{sort keys%k}
[download]

Then I thought "as an excercise, why not do this in Ruby?" Which yielded this 142:

p={}
i={}
ARGV.each{|x|i[x]=1}.clear
while l=gets;l.gsub(/\S+/){|s|p[s]=''if !p[s];p[s]<<$`+$&.upcase+$'if 
+!i[s]}end
p.sort.each{|x,y|print y}
[download]

/s

Update:MeowChow has pointed out Key Word In Context system (golf, anyone)?, which is suspiciously similar, though not identical.