A plume is usually a big, bright, colorful feather (or group of feathers) attached to a pony's bridle. The main exception is the black plume. Black plumes are used for solemn occasions when worn by bio-horses, but can be used for any occasion in human pony play, and they can look pretty hot on a ponygirl.
A plume is solely for show, but that does not mean they can only be worn by show ponies. Any pony can wear a plume (quite a few cart ponies do); it is just a matter of personal taste. The only disadvantages to adding a plume to your pony are:
Personally, I like plumes (well, at least I like them on other ponies). They are anesthetically pleasing, and when matched to the rest of a pony's tack, they add that finishing touch to the whole look...the last touch that draws the eye in and won't let it go.
I have to confess that when I'm the one wearing the plume, I'm a bit more ambivalent than when I'm admiring a pretty ponygirl. When I'm feeling proud, I'm proud to wear it, but when I'm feeling shy, the idea of a colorful feather sticking up from my head seems a little humiliating, and in those cases I put up a fight when T tries to put one on me (I believe she once said that it was like trying to get a toddler into a tux). Anyway, as usual I digress...
You can add a plume to a human pony bridle in a couple ways. Probably the most common attachment method is via a metal plate that attaches to the bridle via the strap that runs across the top of the head (on a bio-horse bridle this is typically called the crownpiece). The metal plate has a screw sticking up from it onto which a plume can attach by simply screwing it on (see photo at right).
This method allows you to easily change out plumes for a desired color, texture, etc. Moreover, the attachment plate for the plume can be added to your existing bridle assuming your bridle has a strap that goes over the top of the head. Also, if you don't want to attach the metal plate to your bridle yourself, Sinvention sells a plume adapter that simply snaps on. Note: Standard bit gag trainers have a strap like this, so you could probably add a plume to one of these.
The only real disadvantage of this attachment style is that it requires all the feathers to already be attached to a central rod (which has a threaded end to screw onto the bridle plate). Thus, you will probably have to buy a separate plume for each color you want instead of simply buying the feathers.
The screw on plumes are usually dyed turkey feathers and 6 inches tall. You can buy taller ones (I've found them up to 12 inches tall), but they are a little more difficult to find (see the bottom of the page for a couple places to find them). You can also find them with ostrich feathers if you prefer the wispier look (though they are a little harder to find).
The second method of plume attachment is via a pouch on the browband of a ponyboy's bridle that has a little pocket for you to directly insert the feather(s) (see photo at right). Obviously you have the flexibility of using different colors without having to buy a ready-made plume each time, which is handy if you see a style or color of feather you like, you can simply buy the feather and add it to your pony's browband.
However, with this method the plume may not be quite as secure and makes the plume more likely to come off the bridle if your pony hits it on something (accidentally or purposefully) or under a strong wind. Moreover, you are limited to putting the plume in the browband as opposed to the crownpiece (as is often done with bio-horses).
Before you buy a plume, make sure your bridle either has an attachment for a plume, or can be fitted with an attachment/adapter. If your bridle does not already have an attachment for a plume, you will need to either buy an attachment plate (some plumes come with one, but double check before you buy) or buy the snap on adapter sold by Sinvention. Although I have not tried the Sinvention adapter, this seems like the best way to go if you can spare the ~$50 because you can easily change a plume between different bridles.
If your bridle already has an attachment for a plume, you're all set - though do make sure the attachment point is the standard screw. If your bridle has the little pouch on the browband instead, you will need to either buy individual feathers, or a different kind of plume (on a thinner rod). However, individual turkey and ostrich feathers are widely available in different colors and sizes on eBay and Amazon.
Listed below are some places to buy plumes with the standard screw on attachment (the retailers, with exception of Sinvention, are vanilla, bio-horse stores):