MacOSX PDF viewer: Automatic reload on file modification

Skim provides this feature. It also provides pdfsync synchronization, so I would consider it as the favorite PDF viewer for LaTeX!

To turn on the feature, go to Preferences in the main Skim menu, then find the Sync tab and from there select Check for file changes and Reload automatically.

One nuisance about Skim is that on the first change it asks you if you really want to reload the document. There is, however, a hidden preference to disable this behavior:

defaults write -app Skim SKAutoReloadFileUpdate -boolean true 

Another potential nuisance: If you compile very large (that is, computation-intensive) documents, Skim sometimes gets out of sync with the file system or even crashes; for details look into this answer.

This is simple with TeXShop installed. First launch TeXShop, and go into the preferences. In the second tab there is a tick box title called Automatic Preview Update. Make sure it's ticked.

Now ok that, and quit TeXShop (you need to quit and reopen to make it honour the preference). Now all you need to do is open the pdf you're working on with TeXShop, and the TeX file in your editor of choice. Any changes to the pdf will make the viewer refresh.

As you're using AquaMacs, you'll probably need to enable SyncTeX (if you want it) in the AucTeX options. There are more instructions on that here.

As OS X has built-in capability of displaying PDF, there are several application based on it. Preview.app is shipped with OS X, Skim is a sourceforge project and TeXShop (mentioned in another answer) is shipped with MacTeX. All of these support automatic reloading. Note that these viewers have a few limitations compared to the Adobe Reader, for example they cannot display layers (OCGs) selectively, they do not execute JavaScript and have a few other problems (see for example the recent question about hyphenation and searchable pdf).