The contentEditable property is what you want - it is supported by IE, Safari (and chrome as a by-product), and I think firefox 3 (alas, not FFX2). And hey, this is also part of HTML5: D
Firefox 2 supports designMode, but this is limited to individual frames, while the contentEditable property applies to individual elements, so you can improve your editable content with your page: D
[Edit (olliej): Remote example, because the contentEditable attribute does not pass SO output filters (despite working in the preview) :(]
[Edit (olliej): I showed a very simple demo to illustrate how he behaves.
[Edit (olliej): So yes, the contentEditable attribute in the associated demo works fine in IE, Firefox, and Safari. Alas, resizing is a css3 function that only webkit supports, and IE does its best to combat almost all CSS. Sigh]
olliej
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