While it's not quite the same as TylerWM, ShiftIt works great and I've been using it for years.

I am actually using my own fork of ShiftIt, where I added being the ability to resize windows to various percentages of the screen: https://github.com/nileshk/shiftit Though my fork is off of an older version of ShiftIt (the original from Google Code), and also pretty specific to my preferences, so I'd recommend checking out the official one first. I really need to update my fork with the latest changes from the official fork, because they added things like definable keyboard shortcuts.

TylerWM looks more like what I really want, as it appears to be more like the tiling window managers you see in Linux, where it dynamically rearranges all the windows on the screen.

Also, I've tried Divvy (not open source) and like it, and you can accomplish the "percentages" thing really easily with it, but ShiftIt has been more reliable for me, so I've stuck with it.

Edit: This was also mentioned in this thread and looks pretty compelling: https://github.com/jigish/slate