In the Apple world it's Barebones BBEdit (it doesn't suck ®) which rules supreme. No Windows or Linux versions available though... shame as it's what I use and it's crackin' :)

Gotta pay to get the full version, but a limited version is available for free.

Update: As Nkuvu pointed out the Lite version is no longer available but a fully functioning Demo version for 30 days is... shame...