I use 'ack' every day to search through thousands of source files spread across dozens of distributions. It beats the hell out of:
find . -type f !-path '*.svn*' | xargs grep Foo
Every time I work on a new computer, I invariably try to run ack at some point and get annoyed that I have to fall back to 'egrep -r '' . | grep -v svn'. I hope that someday it's pre-installed everywhere.