Opera has released a public beta of its next browser release, 10.6 - featuring support for royalty-free WebM video and HTML5 offline web apps. This will be the next major milestone for Linux and BSD users, since Opera is skipping an official 10.5 release for those platforms.
On older hardware it may not matter so much as the overall responsiveness and browser memory footprint. Working on constrained (translation: decrepit) hardware recently it was noticeable how much faster Google's Chrome browser was overall.
Opera split the release schedule of the various platform releases for the first time earlier this year, as it rushed out a Windows version of 10.5 for the Windows "browser ballot" for which it had lobbied. This left the Mac and (especially) Linux versions lagging a little. Both have greater use of native widgets in the 10.5+ builds.
The 10.6 release also adds geolocation and a few more UI tweaks.