Two of my favorite OS X applications are iPhoto Library Manager and AudioHijack pro.
Alas, both, I've recently discovered, use Unsanity's Application Enhance framework (APE) hack.
I don't recall either app ever providing "informed consent" of use of this hack -- though I think AudioHijack might have.
I still shiver when I remember the history of DOS TSRs (terminate and stay resident), and APE is the same sort of thing -- a way to hack applications that are already running . The inevitable result of such hackery is that the applications become less stable .
A less obvious result of this kind of bad habit is that a major OS update can break big time:
... has been the implication of Unsanity's Application Enhancer (APE) framework in some upgrade problems. APE has a long and sometimes controversial history, with some developers swearing BY it (Audio Hijack, for example, uses APE to enable the "Instant Hijack" functionality) and other developers swearing AT it (APE's ability to modify other applications at runtime, necessary to enable some tools, can also make app debugging more difficult)...
... Rosyna of Unsanity sent out an urgent email alert to mailing list subscribers (reproduced in whole below) recommending that APE be updated to the current version (2.03) prior to upgrading to Leopard, lest badness ensue...
The badness is that Leopard blue-screens on install. Archive and Install avoids the problem and that's what Apple is now advising blue-screen victims to do. It's what I prefer to do myself in any case.
One good thing about 10.5, even for those of us waiting for 10.5.1, is that it's going to kill APE. IPLM's author reported that he can do 99% of what he needs to do without hacking 10.5, and I think the same is true for AHP (Instant Hijack isn't essential).
I do wish they'd never used it to begin with.
 In the old days only one application could run at a time, so the TSR was simultaneously hacking the OS and the application. Excuse me while I try to forget.
 Incidentally, Microsoft's sanctioned Outlook plug-ins seems to have a rather similar effect on XP and Outlook stability!
Update 10/29/07: John Gruber has more details. Logitech's "control center" turns out to be a very bad APE offender. I still think it was a bad idea for IPLM to use an APE hack, but at least it was in a good cause -- getting around Apple's missing iPhoto functionality. Logitech had no excuse at all. Friends don't let friends buy Logitech.