Opera 8.0 went gold last week. There are two versions, the free ad-supported version or the pay version ($39US). Within these categories, there are versions for Windows, Mac, Linux (including Xandros), PDAs, and mobile devices.
I'm trying the Linux version and find it substantially faster than Firefox (YMMV). You may also be interested in this comparison between IE 6.0, Firefox 1.0, and Opera 8 that finds Opera is the most standards compliant. Thanks to 18-year old student David Hammond for his work on the comparison.
