MTCOBRA  This software is listed as being "free." Yet when you read the 'license agreement you encounter this:

2 LICENCE KEY

2.1 The use of the software, beyond ‘demo’ mode and any specified trial period, requires a licence key from the licensor. The licence key may be a printable digital key, a non-printable file, or any other mechanism used by the licensee for licence activation purposes. Each licensee, upon payment of the licence fee, will be provided with one licence key which may be used by that licensee only. The licence key will be valid for all point releases of the current major releases. The licensee shall not (i) attempt to “hack,” “crack,” or otherwise override this licence key or (ii) distribute this licence key.

Which is it, Free or Fee?   
(Version 2.6.6)

LYLEHM  Freeware that is well worth a donation. If you need to know what files are being accessed on your system (helpful for so many things) in an easy to use, attractive interface, this is the best I've seen.

Please consider the value you receive and take the time to donate. Developers of such useful apps must be enticed to continue their efforts to all our benefit.

To the fellow that was confused by the toolbar buttons...

Hover your mouse over the button for a popup. You can also right-click and select "Customize Toolbar." There you can either identify all the buttons, customize their arrangement, or permanently add text labels.  
(Version 2.6.6)

JP2110  This is essential stuff. It should be on every Mac. it can solve so many problems, you can't even imagine.  
(Version 2.6.6)

SHOTGUNWEDDINGS  Hands down, the best in it's class.

The only tool of it's kind that visually shows what is being accessed in real-time when launching programs or using an installer.

P.s. The fools that suggested the alternatives are clueless, none of those un-installers have ever visually shown anything for launched apps or installers.   
(Version 2.6.6)

PAUL COOK  Looks cool, but it needs updated documentation.

The help file says that to run it, "hit the green run button..." Well there are two green icons, but neither seems to be a run button. I'm guessing that the run button got replaced by a cd/MP3 player style play button that looks like a triangle pointing to the right.

I still haven't figured out what the other buttons do, so a bit of updated documentation would be helpful. Or if someone knows, posting it here would be helpful too.

But, I have been able to get it to work well enough that I may actually pay/donate $$$ for it.  
(Version 2.6.5)

WALT9500  YES! This is incredibly helpful for any developer or supporter who needs to know what is going on with a hard disk, and who changes what and where.

The user interface is well done, and has some cool ideas.

I love it!  
(Version 2.6.2)

CRUENTATIO  GREAT! :)  
(Version 2.6.2)

GUILLAUMEK  Very useful app. Great interface.  
(Version 2.6)

MM2270  Wow, just wow! I am sufficiently impressed with this app. Amazing graphical view of everything happening on your system. As a systems admin I can see really nice uses of this in troubleshooting misbehaving apps. This one goes into my permanent toolbox.

Great work. And it's free? Too hard to believe!  
(Version 2.6)

PETER DA SILVA  What do the little number badges on the file icons represent?  
(Version 2.5)