Internet Explorer private browsing is fantastic (the only technical advantage over competition left IMHO). Starting new private windows doesn't close the existing tabs. One can have many private windows, each with separate cookies. So logging in say google mail (or HN) with 15 different accounts at the same time is not a problem.
Same exact thing in Opera. Private browsing is per-tab, cookies are separate per private tab (so multiple accounts into gmail is okay). You can open private tabs right next to regular, they don't live in another window (as Chrome enforces).
