(This question is very similar to 10458. It was suggested that Fedora and Ubuntu/Debian are different enough to warrant different answers.)
As I use any Ubuntu setup I gradually install a number of packages over and above the baseline installation. If I reinstall, or if I need to install a new machine, I usually want to reinstall those specific packages, and I want to do it fast to get back to work with a minimum of hassle. As far as I've seen all of the package managers (
synaptic) can tell me which packages are installed, and they all have logs (albeit different ones for each tool, which is a hassle). But none of them can tell me which packages I've installed, as opposed to their dependencies or system updates. Even the logs are tricky in that I'm not entirely sure what I should be extracting from them, or how to integrate them (in the case of the various apt family tools). This means that each time I re-install, or even just backup, I'm not sure how to re-create that list.
I'm not necessarily expecting any of the tools to do this for me, but if they don't I'm looking for workarounds. Even patterns to grep for, good rules of thumb, or a clear idea of what exactly is being logged, would be useful. There may not be a "best answer" here but good ones would be very helpful.
Most of the answers below provide some approximation of what I am looking for, and are useful to some degree. The chosen one is the one that comes closest to a reasonably automatic way of re-installing my tools on a new system, even with all its caveats.