sounding
Sounding (also called stuffing) is a form of urethral play: it is the generic term for the widening of the male urethra using a tool commonly referred to as a sound. Sounds (called uterine dilators in the medical world, see Webster 1913's medical definition of sound) were originally used to help treat venereal diseases back in the day; these days they're used to ready a person for the insertion of a catheter. Urethral play sounds (to which the rest of this node refers) are usually made of steel or steel-plated brass and may be either hollow or solid. The thickness of a sound is traditionally measured by French urological sizing (20 Fr = ¼ inch; 40 Fr = ½ inch) but is also measured in millimeters (26 mm = 1 inch).
Why would anyone want to do this? Pleasure, of course. Some men enjoy the feeling of having something in the urethra. The feeling of urinating is also said to be more pleasurable. It is easily incorporated into "traditional" (male penetration) or more extreme (electricity) sex play but does not have to be; some men practice sounding as a personal growth sort of thing, and others "just because."
The two major types of sounds are Pratts and Hegels. Pratt and Hegel sounds are different: Hegels are shorter (7½ inches) and rounded at the end; this is a good tool for sex play or starting stretching. Pratts are longer (10 inches) and more drastically tapered at the end; this makes for more serious urethral stretching and for reaching the prostate and bladder. Sounds come in a many shapes and sizes: straight, drastically curved at one end, gently curved the entire length or just at the end, S-shaped, or even bent at one or both ends. Once I even saw a vibrating sound for sale. Lubricant is sometimes necessary and always a good idea when sounding; an antibacterial ointment can be used as a precaution against germs. The sound should never be forced further than it easily and comfortably slides, as that's an effective way to cause a very bloody (but thankfully not very serious) injury and can heal into scars or keloids.
Sounding presents few dangers when practiced safely. All sounds should be cleaned before use: if only being used on one person, the sound can be cleaned by wiping it down with household bleach and then thoroughly rinsed; for more than one person, sterilization fluid (such as Madacide) or autoclaving is a good idea. Boiling, microwaving or pressure-cooking sounds can damage the plating, and metal never belongs in the microwave anyway. Washing your hands before starting (or using gloves) is also a great idea as most urinary tract infections are caused by unclean hands.
Somewhat related to sounding is the prince's wand, a piece of jewelry like the love child of a sound and body jewelry for a trans-urethral penis piercing.
References:
BMEzine: http://www.bmezine.com/
Steelwerks: http://www.steelwerks.ca/
Fozzie's Den: http://www.fozden.com/