For me it's the TextFX menu and the MIME Tools plug in. They offer very easy access to lots of handy features like changing case (upper, lower, proper, sentence, invert), remove/swap/replace quotes ('/"), removing trailing spaces, non-printable characters, and blank lines (or just extra blank lines), base64/URL/HTML/ROT13/UU encoding/decoding, all kinds of decimal/hex/binary/octal/text conversions, add/remove escapes, etc.

Having so much there in just a couple clicks is very nice!