Sed is available on all GNU/Linux distributions there is also a versions for Mac OS X too.
Typical usage of Sed are simple text replacement, testing for substrings, complex text replacement with regular expressions.

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cat example.conf 
foo+bar=foo

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sed -i 's/foo/foo_bar/g' example.conf

foo in file example.conf will be replaced with foo_bar

After replacement:

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foo_bar+bar=foo_bar

Start a dry run without changes, you will get a output about the changes (your file will stay untouched):

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sed 's/192.168.0.1/nameserver.local/g' example.conf

Append after match:

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sed '/PATTERN_TO_SEARCH/a APPEND_ME' my_file

Remove line number 134 from file:

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sed -i 134d /root/.ssh/known_hosts

MAC OS X Section!

Sed inplace editing (-i) is a kind different on Mac OS X: in the normal UNIX case you will get a error:

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id$ sed -i '/anna/ d' testfile
sed: 1: "testfile": undefined label 'estfile'

Soo this is the right syntax:

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sed -i '' '/anna/ d' testfile

By the way, this will delete all lines which contains “anna” ;)

From the manpage: you risk corruption or partial content in situ-ations where disk space is exhausted, etc.