How can I list all packages I've installed from a particular repository?

There seems to be no record of the origin of an installed package.

If you are fine with getting the location from whence a package of the same name would be downloaded from, this is available through apt-cache policy. The following (rather ugly) script does the trick for me:

LC_ALL=C dpkg-query --showformat='${Package}:${Status}\n' -W '*' \
  | fgrep ':install ok installed' \
  | cut -d: -f1 \
  | (while read pkg; do 
       inst_version=$(apt-cache policy $pkg \
                                | fgrep Installed: \
                                | awk '{ print $2 }'); 
       origin=$(apt-cache policy "$pkg" \
                          | fgrep " *** ${inst_version}" -C1 \
                          | tail -n 1 \
                          | cut -c12-); 
       echo $pkg $origin; 
     done)

Note that it's quite fragile, as it makes assumptions about the output of apt-cache policy, which might change across versions...

Open Synaptic Package Manager and click the "Origin" button on the bottom of the left sidebar. It will list your sources. Select a source to see the available/installed packages.

Expand the "Installed Software" item in Ubuntu Software Center. You'll see a list of all the different repositories that you've enabled. Clicking on the repo will show you the packages you've installed from each.