So I decided to get the bottom of it, and disabled javascript on all but a few pages. The memory leak disappeared and Opera 12 consumes less than 300 MB RAM (with 32 open tabs).

While Chrome consumes more than 1.3 GB RAM with 10 tabs, and IE around 800 MB with 2 tabs (while flash playing video)…
Opera is still my favorite browser, but it’s unfortunately dying off…