It's also impossible to walk into a perfume shop without stumbling over another celebrity fragrance. Just think of "Intimately Beckham" by former Spice Girl Victora Beckham and "Usher" for Him and Her.
Perfumes can be tricky, however, since fragrance is considered a very personal product. Consumers don't want to compromise their personality too much by taking on someone else's scent, says Julia Beardwood, founder of Beardwood & Company, who has worked with everyone from the model Iman to Calvin Klein.
Male fragrances, especially, can end up looking very tacky, she says, since men tend to be more conservative in their grooming choices. Beardwood says scents from Antonio Banderas, Donald Trump and Prince's 3121 are branding "don'ts."
"Many men like Prince but I don't know if they want to smell like him," she says.
Celebrities often also like to start their own food and drink lines. Comedian Jeff Foxworthy, for example, has launched a line of barbecue sauce and beef jerky, while Tiger Woods put his name on Gatorade's Tiger Focus -- which the company quietly discontinued around the same time Woods' sex scandal broke.
Sports is another big sector: Woods, Michael Jordan and Lance Armstrong have all made millions with their own sporting lines.
There's little question why many celebrities go to the trouble of starting their own lines: money.
"I'm sure you and I would do it if we thought we could make a couple of million dollars by just putting our name on a fragrance," says Bateman. Companies who enter joint ventures with celebrities are often taking a gamble.
On the one hand, celebrities help products stand out: Madonna's name is likely to attract a certain segment of consumers, convincing them that those sunglasses -- more than hundreds of other styles that might be available more cheaply -- will give them a certain flair.
Plus, the media exposure that a celebrity can offer is priceless. When Iconix released news of its joint venture with Madonna, the company's stock price bounced about 1 percent.
But celebrity scandals can cost a brand dearly, if consumers continue to associate that brand with the star's indiscretions.
"The question is whether all the Lindsay Lohan stuff is good for Ungaro," says McCormick. "Will people see the Ungaro name and only remember the Lindsay Lohan story, or is Ungaro later going to see the benefits of having had their name in the press?"