Before every release, we create one or more "release candidates" for adventurous people to test first.
During development cycles, many people test intermediate builds. These are built every night automatically and made available to all our users. This helps finding bugs very early so they won't even get into an official release.
A big user community helps out with testing each build before we release it.
A custom crash report tool is included in every TortoiseSVN release which helps us fix the bugs much faster, even if you can't remember exactly what you did to trigger it.