If I had to name one utility no Mac OS X power user should be without, it would be Charles Srstka's Pacifist. Well, okay , it might be Alsoft's DiskWarrior. But then Pacifist would certainly come in second! In simple horse-race terms, counting the number of times each program has rescued me in a tight spot, it would be a photo finish.
In simple terms, Pacifist lets you look inside .pkg files. A .pkg file is the sort of thing you double-click to start up the Installer utility; in fact, you might well think of it as being, itself, an installer. The trouble with such installers is that you may not know what they will put where, or you may not be given enough choice about which of its contents you want installed. With Pacifist, you can browse a .pkg file as if it were a folder; you can see its contents, learn a lot about what it proposes to put where, and extract individual files.
To give a practical example, just last week I saw a note on a Usenet newsgroup from someone who had somehow damaged his copy of iMovie. Following advice from other readers, he found iMovie inside a .pkg file on one of his system installer disks, and was easily able to extract it to his hard disk without running the installer.
Pacifist can also read Receipt files (lists of what was installed where, left on your computer by the Installer), and thus can verify that things are properly installed, with correct permissions and so forth. A nice use of this feature is Pacifist's capability to tell you where your .kext files came from (choose Display Kernel Extension Report from the Pacifist menu). A .kext file is a kernel extension; these files are crucial because they modify the operation of Mac OS X at a low level, so a buggy one can cause things to go mysteriously wrong. In this way, I see instantly that I have just two .kext files that weren't installed by Apple, and I know what both of them are for, so all is well.
What has always most struck me about Pacifist is its generosity. It does something difficult and technical and makes it easy and safe, along with a crystal-clear interface. Despite all the work that must have gone into writing and testing and maintaining it, Pacifist 2.0 is still just $20, and is true shareware: if you don't pay the fee, the only penalty is a nag screen at startup. This new version, besides fixing some bugs, adds full Tiger and Intel support, and allows reading of additional file formats (including .dmg). For registered owners of an earlier version, this upgrade is free.