Delphi is a professional editor and compiler for creating Windows applications, based on Object Pascal. Available as part of the monolithic Delphi Suite or as the more compact Turbo Delphi, it's my tool of choice for working with this OS. See their website.

Lazarus is a free, multi-platform, open-source development environment much like the earlier versions of Delphi. It's based on the superb Free Pascal compiler, and most Delphi projects can be modified to compile under Lazarus with a little effort. It's a tad less stable, and in some ways incomplete, but for independent developers it's great. See their website.