In the past, I have written about “Opera equivalents to Firefox extensions”:http://virtuelvis.com/archives/2005/01/opera-and-firefox-extensions – with a “follow up”:http://virtuelvis.com/archives/2005/09/opera-and-firefox-extensions-ii.
Rijk has taken this one step further, and surveyed “150 Firefox extensions”:http://files.myopera.com/Rijk/blog/extensions.html. Do these extensions have equivalents in Opera? Mostly, they do, either through built-in functionality or simple customization, UserJS or widgets.