The Best Man or the Best Woman
Traditionally, the groom in a wedding party will choose a man in his life that is important to them, possibly a brother or a very close male friend to be the best man. Nowadays, while traditions are still going on on a regular basis, many groomsmen are choosing a woman to stand by their side. It really doesn't matter whether it's the best man or the best woman, as long as the groom has someone who can assist them on the wedding day.
Typically, the best man, best woman, or honor attendant, will have several different types of duties throughout the wedding. This can be as simple as standing beside the groom while he gets married, or as complicated as handling bachelor parties, being legal witness to the marriage, making toasts at the reception, handling the bachelor dinner, helping with the honeymoon, and many other chores.
If it's a woman for the best man honor, the attire can be a bit confusing. It's best to speak with the bride as well as the groom in order to determine the best woman's wedding attire. There are many choices that can be made from matching the bride's colors in a dress or a suit, to wearing a tuxedo. And it's all according to how the wedding party would like to be presented.
In today's world traditions are often tweaked just a bit according to the lifestyle of the bride and groom. Don't be afraid to be a bit different, there are been many situations where two fathers walked the bride down the aisle, brothers or best friends walk the bride down the aisle, as well as a variety of other adjustments to the tradition. If you're best friend is a woman, and your bride gets along with her, there's no reason you can't have the best woman instead of the best man.
Don't be afraid of what people will think or do, most people who go to a wedding just want to see the bride and groom as happy as possible. Traditions are great, and many people enjoy traditional weddings, but tweaking the wedding a bit to make yourself happy is your choice. The best thing you can do if you're considering tweaking your wedding just a bit is to make sure those people who are important to you understand the reasons why. Those who care about you easily understand using a best woman instead of a best man and there's a good chance they already know the woman anyway.
A woman or man can do the best man duties, making yourself and your wedding something to remember by having your closest friends by your side is what counts, not what gender the individual participants may be.