Jay Leno has been hosting the “Tonight Show” for 15 years on NBC, and he’s had the No. 1 show on late night for nearly that long. To celebrate Leno’s official 15th anniversary tomorrow night, NBC is doing something that would have been impossible back when he debuted, before the internet blossomed.

The network has invited viewers to create their own “Tonight Show” promos and music videos through NBC.com. The site is offering up clips from the past decade and half, along with graphics and sound effects.

The videos are then posted on NBC.com for all to see. As of yesterday afternoon, there were more than a dozen.

It's another example of how, in this latest television age, online has gone from a catchphrase to an integral part of programming, and perhaps no network has been as aggressive or as imaginative as NBC.

While it may be the No. 4 network in terms of viewers, NBC has been well out ahead when it comes to creating online extras to draw people to its web site.

Already this season, it has added extensive online elements for its hit show “Heroes” and launched a new microsite for the popular annual “Where in the World is Matt Lauer” segment on “Today.”

It appears to be working.

According to Hitwise, the online tracking firm, NBC.com had the largest market share of visits to broadcast network sites in March at 37 percent, 7 points higher than ABC and more than double that for CBS, Fox or CW.

NBC.com visits were up 10 percent versus the previous month.

That’s all the more remarkable considering NBC has just one top 20 non-sports show this year (“Heroes”) among adults 18-49, while ABC, Fox and CBS all have at least three.

There's more to come. At its upfront presentation last week, NBC introduced an online component to each of its new shows.

Example: “Lipstick Jungle,” the “Sex and the City” knockoff about four career women, one of them the editor of Bonfire Magazine, will launch an online version of the faux magazine with iVillage, the women’s portal owned by NBCU.

The faux magazine will contain real editorial content typical of women’s magazines like Marie Claire and Glamour.

The network will also launch a new online-only soap called “Coastal Dreams” and added features such as horoscopes for the characters on its TV soap “Days of Our Lives.”

Meanwhile, in other dayparts for the week ended May 13, NBC’s “Tonight Show with Jay Leno” was first for the week in late night, averaging 5.4 million total viewers and a 1.7 rating among adults 18-49. “The Late Show with David Letterman” on CBS had 4.4 million viewers and a 1.4 rating in the demo, with ABC’s “Nightline” bringing in 3.5 million viewers and a 1.1 rating. In late late night, NBC’s “Late Night with Conan O’Brien” had 2.2 million total viewers and a 0.9 in 18-49s, with CBS’s “The Late Late Show with Craig Ferguson” averaging 2.0 million viewers and a 0.7, ABC’s “Jimmy Kimmel Live” at 1.8 million viewers and a 0.6 and NBC’s “Last Call with Carson Daly” bringing in 1.3 million viewers and a 0.5 among 18-49s.

NBC’s “Meet the Press” was again first among Sunday morning shows in total viewers, bringing in 3.32 million, and among adults 25-54 with a 0.9 rating. CBS’s “Face the Nation” was second in viewers with 2.72 million and second among 25-54s with a 0.8 rating, followed by ABC’s “This Week with George Stephanopoulos” with 2.66 million and a 0.7, and Fox “News Sunday” in fourth place with 1.2 million and a 0.4 among adults 25-54.
 
In morning shows, NBC’s “Today” was first for the week with 5.5 million total viewers and a 4.3 household rating and 15 share, followed by ABC’s “Good Morning America” with 4.9 million viewers and a 3.8/14. CBS’s “Early Show” was third with 2.6 million total viewers and a 2.1/8.
 
In daytime, CBS had the largest audience for both daytime dramas and full daytime, 3.7 million and 3.96 million, respectively, but was second among women 18-49 for daytime dramas with a 1.5 rating and full daytime with a 1.4. ABC had the second-largest audience with 3.36 million watching its dramas and 3.46 million for full daytime, and was first among women 18-49 with a 1.7 rating for both dramas and full daytime. NBC had an audience of 2.14 million for both dramas and full daytime, and was third with a 1.1 rating in women 18-49 for both.
 
In evening network news for the week ended May 20, ABC’s “World News with Charles Gibson” was No. 1 among total viewers and 25-54s for the fourth straight week, leading with an average of 8.05 million viewers and a 2.1 25-54 rating. NBC’s “Nightly News with Brian Williams” was second, averaging 7.16 million viewers and a 1.7 among 25-54s, while CBS’s “Evening News with Katie Couric” was third for the week with 6.17 million total viewers and a 1.5 25-54 rating.