Willie Nelson may be the most popular duet partner in country music history. In fact, his first Top 10 hit -- 1962's "Willingly" -- was a duet with Shirley Collie, who became his second wife.

More than 50 years later, he's still happy to harmonize. His legions of fans can look forward to an upcoming episode of CMT Crossroads with guest stars like Sheryl Crow, Jamey Johnson, Norah Jones, Ashley Monroe, Leon Russell and Neil Young singing along.

Nelson used to celebrate his birthday on April 30, until one year his mother mentioned that he arrived in the world just before midnight -- a few minutes before the doctor looked at the clock. Taking that story to heart, Nelson will officially ring in his 80th year on Monday (April 29).

His friends at Farm Aid have also established a website where Johnson, Jones, Young, Kenny Chesney, John Mellencamp, Dave Matthews, Sting and many other celebrities are offering their birthday wishes via video messages.

Looking back on a celebrated career of collaborations, here are 10 prime Willie Nelson duets.

"Beer for My Horses" With Toby Keith

This tune about Old West-style justice needed a duet partner with a sense of humor and a strong connection to the past, making Nelson and Toby Keith the perfect pair. Channeling his inner lawman, Nelson's vocals added a bit of believability to all the talk of stringing up bad guys, but he conveys it in a fishing tale kind of way. When grandpa talks about the whopper he caught back in the day, you know it's mostly true, but probably a little exaggerated. Co-written by Keith and Scotty Emerick, "Beer for My Horses" spent six weeks at No. 1 on Billboard's country songs chart and eventually spawned a feature film of the same name.

"Faded Love" With Ray Price

The sentiment of Waylon Jennings' "Bob Wills Is Still the King" still holds true in Texas. With that in mind, what could be more appropriate than Nelson and fellow Texan Ray Price teaming up for one of Wills' signature songs? Crystal Gayle provides background vocals on the track that reached No. 3 in 1980. The collaboration would have probably done even better if Nelson's label had not released a solo single -- "On the Road Again" -- just a few weeks after "Faded Love" first charted.

"Good Hearted Woman" With Waylon Jennings

Some would say this 1975 anthem embodies the Outlaw spirit. But rather than being intimidated by Waylon and Willie, fans embraced them. The single spent three weeks at No. 1 at country radio and won a CMA Award. Legend has it Jennings came up with the song idea while playing pinball and that Nelson suggested one of the lines at a poker game. The corresponding album, Wanted: The Outlaws!, proved to be just that -- wanted. In 1976, it became the first million-selling album in country music history.

"Heartbreak Hotel" With Leon Russell

Russell's career has taken a lot of twists and turns from his success as a Hollywood session musician in the '60s, through his rock star status in the '70s and, in 2010, his work with Elton John on their album The Union. He also recorded several country albums, most notably 1973's Hank Wilson's Back. Six years later, he and Nelson recorded One for the Road, a collection of country classics and American pop standards. By speeding up the tempo on "Heartbreak Hotel," they won't make you forget Elvis Presley's original recording, but their energetic interpretation is a lot of fun.

"Just to Satisfy You" With Waylon Jennings

Don't let the title deceive you. Willie and Waylon are not singing "Just to Satisfy You" to a good-hearted woman. You can tell that much from the first line: "Someone's gonna get hurt before you're through." This gal is well-known for leaving a trail of broken hearts in her wake -- and the guys seem to be singing from experience. Nearly 20 years after Jennings wrote it with Don Bowman, this 1982 duet became Willie and Waylon's third No. 1 single together.

"Mammas Don't Let Your Babies Grow Up to Be Cowboys" With Waylon Jennings

Arriving in 1978 on the album Waylon & Willie, "Mammas Don't Let Your Babies Grow Up to Be Cowboys" proved to be career-defining for both performers. Although Jennings passed away in 2002, Nelson rarely plays a show without including "Mammas" at some point. Written by Ed and Patsy Bruce, "Mammas" appealed to male and female country fans by conveying the rugged individualism of a cowboy, as well as the sensitive side. The song spent four weeks at No. 1, earned Nelson and Jennings a Grammy and currently lives on in smoky old pool rooms everywhere.

"Mendocino County Line" With Lee Ann Womack

Coming from Nelson's 2002 album The Great Divide, "Mendocino County Line" stands out for two reasons: its contemporary country production and Lee Ann Womack's silky smooth voice. That's not to say Nelson was an afterthought on his own song, though. His delivery is tied perfectly to the emotion of the song which finds a couple reminiscing fondly about a time when they were deeply in love. Written by Matt Serletic and lyricist Bernie Taupin, "Mendocino" only made it to No. 22 on the Billboard country songs chart, but it did earn Nelson and Womack a Grammy award for best country collaboration.

"Pancho and Lefty" With Merle Haggard

Nelson and Merle Haggard's version of the Townes Van Zandt classic "Pancho and Lefty" is pretty much the standard interpretation, even though biographers say Van Zandt never could explain what it was actually about. Pancho Villa? A pair of drifters he met in Texas? Nothing at all? The narrative itself is clear enough: Two weary bandits are on the run in Mexico when one of them sells the other out for a reward. Released in 1983, Nelson's sorrowful vocals never push much beyond delicate disappointment. He just lost his best friend and his life, after all. "Pancho and Lefty" spent one week on top of the Billboard country songs chart and remains one of Nelson's most enduring recordings.

"Seven Spanish Angels" With Ray Charles

Nelson's longtime friend actually got top billing on this track from 1984 that became Charles' only No. 1 country hit. It was featured on Charles' Friendship album that also included collaborations with Hank Williams Jr., George Jones, Merle Haggard, Johnny Cash, Ricky Skaggs and the Oak Ridge Boys, among others. Written by Troy Seals and Eddie Setser, the Western imagery takes on an even more cinematic quality through the production and, of course, two iconic voices.

"To All the Girls I've Loved Before" With Julio Iglesias

My friend's mother really loves this song, too, which surprised me at first. You'd think being one of "all the girls" wouldn't be much of a compliment. However, rather than being lumped into a pile of exes, the former flames are celebrated for everything they brought into the guy's lives. And instead of drunk-dialing, these gentlemen are content to reflect on bygone romance. The swooning single crossed over to Europe, Australia and South Africa and topped the U.S. country chart in 1984.