pbpaste | some_command | pbcopy

For example:

• Converting spaces to tabs, for pasting into a spreadsheet program: pbpaste | tr ' ' '\t' | pbcopy

• Converting tabs and newlines to html table formatting: pbpaste | sed 's/^/<tr><td>/' | sed 's/\t/<td>/g' | pbcopy

• Escape angle brackets and ampersands for html: pbpaste | sed 's/&/\&amp;/g; s/</\&lt;/g; s/>/\&gt;/g;' | pbcopy (I used this on itself before pasting into this post.)

• Convert newlines-indicate-paragraphs text to html: pbpaste | sed 's/^/<p>/' | pbcopy (I use this in putting together the kids text posts.)

• Any time I want to do find-and-replace when working with software that doesn't support it well.

The way this works is that pbpaste reads from the clipboard as plain text, the intermediate command(s) transform the text, and the pbcopy sends it back to the clipboard.

While these are the Mac commands it should be similar elsewhere. For example, on Linux I'd use xsel, though that does require paying attention to whether you want PRIMARY (xsel) or CLIPBOARD (xsel -b).