Extend Path Plug-in can manipulate spirals in Illustrator as well as other paths. The VectorScribe set comes with a selection of plugins (the set is available via the Astute Graphics site along with 20 or so other plugins as part of their subscription). Once you have added the tool, you can select it and then modify any curve very easily
You can access a variety of options which can all be accessed via the tools itself by pressing return or double clicking the tool entry. You can then set the option to 'spiral' and set to a fixed radius etc via that panel.
Create a basic line. If you go for the default, you can just increase the line or decrease it, which is great. If you want a small change to the end of the line, go for a winding constant of 0.2 or a low value and fixed radius of 20pt or a low value. If you want a larger winding design then set that to 0.6 and set the fixed radius to 200pt etc and that will create a much larger curve at the end of the line. You can apply this to both ends of the line. Simply drag away from the line and follow the 'preview curve' and you will see your line increase, and once you are happy with the design, release. You can also go to the other end of the line and apply the same, perhaps increase the line more or less. If you go back to the same design later, and use the same tool, the same preview curve will be there. You can, of course, go to the options again by pressing return and changing the fixed radius etc. You can also then select the reverse curvature option and the curve will not lose the previous addition but will now curve out in the opposite direction based on the current value of the winding and fixed radius. This can be repeated over and over and you can fill the entire design with a swirl / embellished artwork all from a basic line.
You can select any of the generated designs and apply a width profile or change the stroke. You can also change the color all via the control bar if you select the selection tool. You can also apply live effects such as 3D etc to the artwork. You can apply any of the width profiles as well as use the associated tool to modify the stroke. Once you are happy with your design you can go to the object menu and expand appearance. You may like to then use the smart remove tool as you may have a design with lots of points.
Select something like the arc or curvature and create a curve and then you can apply the tool to that with a winding of 0.2 (keep it a low value) and constant radius of 1000pt etc and you can just create a lovely arc design with the tool if you release the mouse before it goes too far along the preview curve, you don't need to follow it all the way along that preview, you can release at any point.
The larger the radius setting, the more options you have for creating all kinds of weird and wavy designs as each time you go back to the path, a new preview will be displayed. When you are using low settings, you will most likely end up with a tight curve at the end of the line with little flexibility to see any space between the arms of the design (such as fixed radius of 20pt etc)
Go to a line or open curve (created using the curvature tool) and apply it with a setting of winding 0.5 and fixed radius of 500pt and then go to the effect in the effect menu and distort such as zigzag. Create an intense crinkled design by using a large ridges setting and size of about 30 etc. You can still go back to the tool and increase and decrease the curve and as you do that, the zigzag will change. You can either expand appearance or if you want to remove the zigzag, delete it via the appearance panel.
Select a curve and go to the effect menu and distort and transform and use the transform. Set the copies to 10 and set the scaling to 0.99 and move to 20pt etc and then go to the tool and apply the selected item and as you do that, all the instances will also be modified. You can also rotate and transform the design and then continue to increase or decrease the curve and all the instances will change. You can press return and change the fixed radius and winding constant to create new variations in the curve as well as use the reverse curvature option.
You can use it with a selected curve as part of a repeat object. Select a curve and go to the object menu and repeat and radial and then modify the number of instances as well as the radius (control bar or interactively). You can't change the line with the tool at this point. You need to double click the radial object and then go to the selected line and increase or decrease using the current winding constant and fixed radius. You can also use the selection tool to scale and rotate the artwork. Exit out of the radius repeat object via the top left controls
You can use a fixed radius but you also have a match radius option and scale radius. If you use match radius with a straight line you will just get a straight line. Same if you use the scale radius, it will still be a straight line. You can use those two first options if you have an open curve. If you want a very tight winded design then set the scale super low. If you have a very tight winded design already and go for the match radius option then it will just make it even tighter. Personally, I use the fixed radius option but it really depends on the end result you want, a less wound design or a super tight design.
You cannot use it with a closed path but you can always use tools to break a design and then you can use the tool to manipulate that curve.