In X (Xfree86 on Linux or UNIX) I could move a window around the screen by holding down a modifier key (the CTRL key if memory serves me correctly) and move a window around the screen by dragging it from anywhere in the window (not limited to the title bar.) Is there a similar functionality available in OS X?
Although it's a paid-for utility (which comes with it's own PrefPane etc), this is a great tool for chucking around your windows. You can also set it up to force windows to certain sizes on a shortcut, which is handy if you want to check out your web pages in a browser at 800x600, etc.
By default, you jut press CtrlCommandup-arrow and you get a little black pop-up window that helps you to move/resize the window that had focus when you pressed the shortcut. (the screen grab includes the PrefPane too).
No, there isn't. The only thing that's possible is to drag a window that doesn't have focus using the Command key. In this way you can move windows in the background without changing focus.
It's not exactly what you are looking for but in Lion you can now hold your mouse over the edge of a window and your cursor will turn into either a vertical or horizontal resize cursor (a new feature so you don't always have to use the bottom corner to resize). However, if you click and drag your mouse in the opposite direction of the resize cursor, the entire window will move with no need to use the top title bar.
Have you looked at BetterTouchTool?
It boasts lots of other features, most notably complex trackpad gestures that can be used to call all sorts of built-in functionality, but does provide the ability to press and hold Ctrl to move a window with the mouse, without even holding left-click down.
The site says its only for Snow Leopard, but a Lion version is in the works (alpha is available, I think as of 0.73). Your particular feature, if that's all you enable, works fine in Lion.
Also Check these answers for other interesting OS X WM alternatives.
I got annoyed that the only options for this aren't free and come bundled with a bunch of other features that are useless to me, so I wrote my own.
A utility for emulating XFree86-style Meta+click and drag window movement. The default bindings are Cmd-Shift-Click for window movement, and Option-Shift-Click for window resizing.