Is this possible in Mac OS 10.5 ?
Not by default, but with Afloat, you can do the same thing, just a different shortcut. With Afloat, you use Control+Command+Drag.
Sorry, too low of a rep to post a comment for clarification, so I'll have to do it in this.
Can you clarify what you mean? For instance, one could interpret your question and say it's been possible to move windows by a left-click and drag on the window's title bar since well before OS X 10.0. In other words, why in the world would you want to use Alt (Option)? (Or what difference would that make?)
Or, are you referring to being able to, from the app you're currently in, move or resize the window of an application that is in the background without making that application active? If so, you can do that by holding down the Command key and dragging the window to move it, resize it, etc. That has been around since at least 10.2. You can also use this method within the same application to move and resize a window without bringing that window forward.
(You can also hold down the Command key and click on buttons or controls in a window that's in the background (even in a different app) and have that click be performed in the corresponding application without that application being brought to the front).
I prefer HyperDock für this task. You can customize the Shortcut as you desire e.g. ⌥+Click.
In Addition there are other nice things that HyperDock can do:
- Preview and select windows & tabs by moving the mouse on a dock item.
- Use shortcuts to open new windows and documents with click.
- Window Snapping
Just check out the Trial and if it meets your requirements, $9.95 USD isn’t that much.
Afloat has moved to GitHub: https://github.com/millenomi/afloat It seems to still work with OS X 10.8, though no development has undergone since 2 years...
[Edit: sept 2017] An even better solution is mySIMBL (https://github.com/w0lfschild/mySIMBL), a package manager to install and load many SIMBL plugins, including Afloat.