I use two.
Skim ($0) (available here) works as a general PDF viewer and has a decent auto-update feature. Occasionally the update is broken (some errors in code compilation cause this at times), but for the most part it is pretty reliable. The window comes to the foreground when it updates, too, which can be nice if you have buried it since the last update.
Latexian is more of a full development environment ($10) (featured on SE) that has a built-in live preview; it has support for bibtex, indexes and glossaries, among other features. As an additional note, I find Latexian to be very useful in prototyping solutions to some of the questions I examine from TeX.SE. It is very quick to start a document, and you can immediately live-preview, even before committing to saving the file(s) on your hard drive.