TeXShop will open any PDF file dropped on its icon, and you can copy and paste any rectangular section of a PDF file into, for example, OmniGraffle, Keynote or PowerPoint. Even easier, you can drag across a section of the PDF file using the selection tool in TeXShop and then drag-and-drop that section into a document from another application. (Drag and drop to the desktop produces a PDF file called "texshop_image.PDF".)
This is incredibly useful if you're adding equations to a diagram: just typeset all of the equations using LaTeX (for example) in a single document, and drag and drop the individual portions whereever they're required. As true PDF files the resulting insertions are resizeable without a loss of clarity. PowerPoint presentations containing equations are incredibly easy to manage, and of course the output that TeX/TeXShop produces is vastly superior to that from, for instance, equation editor. Keynote presentations also accept dragged PDF documents. This sort of capability has previously required a separate application (such as "LaTeX Equation Editor").
If you don't have a TeX installationm there are clear instructions on Richard Koch's TeXShop page (linked above). It's a big download, but the quality of the documents produced using the resulting packages make it well worth the effort. The PDF manipulation mentioned here is really the icing on the cake. Finally, if PDF isn't your favoured format, the dragged sections can be exported/dragged in either PNG, JPG or TIFF format.

