I was not entirely happy with the existing scripts for doing this, so I ended up writing my own.
The script itself is a bit simpler then the others for a couple reasons:
1) I did not write or include any other functionality besides window dragging
2) The logic for moving the window is a bit simpler over the other scripts I've seen
3) IMHO, the script is commented and formatted a bit better.
Anyhow, nothing great.

Code:
;-------------------------------------------------
; Window dragging via alt+lbutton -
; Author: Lasmori (email AT lasmori D0T com) -
;-------------------------------------------------
!LButton::
original_win_delay := A_Win_Delay
CoordMode, Mouse, Relative
MouseGetPos, cur_win_x, cur_win_y, window_id
WinGet, window_minmax, MinMax, ahk_id %window_id%
; Return if the window is maximized or minimized
if window_minmax <> 0
{
return
}
CoordMode, Mouse, Screen
SetWinDelay, 0
loop
{
; exit the loop if the left mouse button was released
GetKeyState, lbutton_state, LButton, P
if lbutton_state = U
{
break
}
MouseGetPos, cur_x, cur_y
window_x := cur_x - cur_win_x
window_y := cur_y - cur_win_y
WinMove, ahk_id %window_id%,, %window_x%, %window_y%
}
SetWinDelay, %original_win_delay%
return
;-------------------------------------------------