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A quick adjustment of the timeout values for key processes can greatly speed up otherwise laggy shutdowns in OS X.
If your organization uses Citrix services, then the background programs for this service may be a cause for some slowdowns.
New malware for OS X is making the rounds in security circles, called "Pintsized," but has not yet been determined to be much of a threat.
Tackling third-party launch agents and daemons may help odd system pauses after an OS update.
If you need to search for files in OS X, one option it is to use the OS X Terminal application and some of its services.
In some situations you may find that iTunes no longer recognizes your iOS device after you restart your Mac running Mac OS X 10.6.4 Snow Leopard. The solution may be more complex than just reinstalling iTunes.
Disabling and re-enabling Spotlight indexing of the drive may help.
If a background process does not launch correctly in OSX, you may see a "throttling respawn" message repeating in the system console. Here are some approaches for fixing this.
Apple has released a knowledgebase article (reprinted here) that covers details regarding DNS server handling in 10.6 and the 10.6.3 update.
Adobe's new updating process should keep Mac users running the latest version of Adobe's popular Web plug-in.