For viewing diffs, I use the excellent Meld tool. But when it comes to looking at earlier revisions, I usually just find myself looking at the branch in Launchpad, where my code is hosted. Are there are good desktop tools for allowing me to do this? Ideally, Olive's "History Mode" should do the trick, but it just gives me errors when I try to switch to History Mode.
I prefer Gtk tools, but I'm open to anything.
I've not been that impressed with the Gtk tools, but the Qt tools for Bazaar are excellent. Try Bazaar Explorer with QBzr: they're very usable and give a good means of looking through the history.
To install the latest versions with Bazaar, do this:
cd ~/.bazaar/plugins
bzr branch lp:bzr-explorer explorer
bzr branch lp:qbzr
bzr-gtk plugin has visualize command or just vis. Also QBzr has qlog command. The latter is a bit more powerful, but requires PyQt4/Qt4 libs.