I use Notepad++ as my default text editor on all my systems, and it has the nice bonus of having this built-in - just go Plugins -> Document Monitor -> Start to monitor. You can also use File -> Reload from disk to manually reload it.
I love the Kiwi Log Viewer. They were just bought out by Solarwinds -- I assume the product is still for sale (or maybe they're giving it away for free now? They're giving away a few other Kiwi products).
I typically do this with TextPad. It has the option to monitor that a file has been modified. It is an option to either automatically update the window or to prompt you whether or not you want to reload the file.