United Way has a PSA with music that sounds a lot like "Viva La Vida" by Coldplay.
A car salesplace in central Florida called Courtesy uses the Ghostbusters theme, but with changed lyrics, in their ads. The lyrics? "Need a car, truck or van? Need a car? Call Courtesy!"
A commercial for gold coins has a recognizable parody of Morricone's "Ecstasy of Gold" (Tuco in the graveyard), while yelling the word "GOLD!" repeatedly.
Frito-Lay created a soundalike for Tom Waits' "Step Right Up" as part of a Doritos marketing campaign. It was close enough that Tom Waits called his lawyer, and ended up making more money in damages than he'd made from all the albums he sold up to that point.
For some reason, the theme from the Old Spice commercials opens with two bars of "Scotland the Brave" and then goes off in a completely different direction.
There is an American Express ad with a backing track that sounded suspiciously like Electric Light Orchestra's "Mr. Blue Sky".
Commercials for proactiv.com, which sells facial cleansers, have a song in the background that sounds suspiciously like Bruce Hornsby's "The Way It Is".
The Danup ad in Mexico features a Suspiciously Similar Song to "Good Vibrations" from The Beach Boys.
As do those Aussie commercials for the Good Guys.
A chicken noodle soup commercial included a song which has a chorus very similar to the part of Green Day's "American Idiot".
The music used in a local Public Service Announcement about water conservation sounds exactly like The Monkees' "Daydream Believer". They get around it by having voices sing a wordless riff every once in a while that presumably counts as an original melody, but if you know the lyrics you can sing along with the music without missing a beat at all. The structure and chords are identical.
Here's an interesting one: Listen to ES Posthumus's song "Pompeii", used in quite a few movie trailers. Then listen to Pfeifer Broz' "DNA Reaction", a song made especially for movie trailers — particularly the latter half.
...and X-Ray Dog with their Pirates' Curse. Seriously, both those companies apparently thought that sooner or later someone's gonna make a pirate movie and will need a recognizable music for the trailer licensed quickly.
There is a Kraft Dinner ad with a teen listening to an MP3 player. The player is playing the first few notes from "American Woman" by The Guess Who over and over again.
The Mighty Mighty Bosstones sued Pizza Hut over a commercial that used an instrumental backing that was suspiciously similar to "The Impression That I Get" at the height of the song's popularity.
A commercial series from Fido has a light and breezy background piece that sounds strikingly similar to Chrono Trigger's "Guardia Millenial Fair".
A Commercial for Barbie dolls uses a song that's blatantly based off Aqua's "Barbie Girl".
Several, some of which keep the "Come on Barbie/Let's go party" part. It's ironic that a song they initially sued over now figures heavily in their advertising.
At least in Sweden, this was a common practice in the mid-to-late 90's. In 1995, a pasta commercial featured a score very similar to Scatman John's "Scatman", as well as visuals in the style of the music video. One year later, an ad for a sailing company used a Suspiciously Similar Song to Peter André's "Mysterious Girl". Finally, the chorus of Doctor Bombay's "Calcutta (Taxi, Taxi, Taxi)" (1998) became the "100 000, Uppsala Taxi" radio jingle in 2000 or so (and remained in use for many years, to the point where most people had probably forgotten the source of inspiration).
The music in the notorious "You Wouldn't Steal A Handbag" anti-piracy bumper is a ripoff of "No Man Army" by The Prodigy. Ironically.
NBC is airing a romantic comedy film it made or Hollywood made. Time to drag out the wacky Suspiciously Similar Song to Katrina & the Waves's "Walking on Sunshine" with the most obvious key change in history to promote said film. Very common in the 1990s.
At the Apple Computer 1984 shareholders meeting, they played a corporate video, "We Are Apple", which rips off the song "What a Feeling", and also includes the lyrics "What a Feeling" in the song itself!
This is incredibly common for company instructional or inspirational videos. Circa 1999, KFC employees were subject to Colonel Sanders voiced by Randy Quaid getting down to "Here Comes the Colonel!" (to the tune of "I've Got the Power!" by Snap!) before a video lecture about food safety.
An ad for the Australian version of the dating site eHarmony has a Suspiciously Similar Song to "Where Is The Love?" playing behind it.
However, if this was the ad in question, it was more of a Sampled Up version of that and "(I've Had) The Time of My Life"
Ironically, The Black Eyed Peas actually did end up doing a song that sampled said song.
More recently, they released a commercial with a suspiciously similar song to Katy Perry's "Teenage Dream."
An Air Force Reserve ad aired during the 2010 Super Bowl used an instrumental that was noticeably similar to The White Stripes "Fell In Love With A Girl".
The trailer for the film Army of Darkness uses a Suspiciously Similar Song to Metallica's Sad But True, Holier than Thou, and Orion.
A Best Buy commercial had a song with the lyrics "I'm free, I'm free, not just a puppet on a string, no ties to anything", which was suspiciously similar to the Cowsills' "The Rain, The Park, and Other Things".
One Dodge commercial used a guitar song that sounded suspiciously similar to "Me and Julio Down by the Schoolyard" by Paul Simon.
One pawn shop ad closely parodies Run DMC's "King Of Rock" as "King Of Gold" - the chief difference being a slightly altered guitar riff.
This Specsavers ad features popular UK kids' character Postman Pat. However, the music in the background isn't actually the original theme everyone knows- and if one listens it's melodically not that similar. However, in the background of a short ad, the style is similar enough that most people probably won't notice- which was probably the intention and still qualifies it for this trope.
An ad for Hidden Valley Ranch uses a mostly instrumental (save for some wordless vocals) Suspiciously Similar Song to The Monkees' "Pleasant Valley Sunday". Since the song has the word "valley" in the title and chorus, it's possible that they originally set out to use a full-on Repurposed Pop Song version.
The trailer for Guvyer II: Dark Hero has a Suspiciously Similar Song to Trevor Jones's score to The Last Of The Mohicans.
A commercial by T-mobile uses a Suspiciously Similar Song to Jason Mraz's "I'm Yours" in the background. It's on the piano, and just a few notes are changed here and there.
The actual brand escapes memory, but one auto maker has a commercial which uses a soft arrangement of Tool's "Vicarious".
A post-Billy Mays OxiClean ad (featuring Anthony "Sully" Sullivan) plays a Suspiciously Similar Song to "Hit Me With Your Best Shot" by Pat Benetar through the spot. They used the song in another OxiClean ad.
A commercial for the Chevrolet Tahoe in 2006, directed towards a more urban audience, had a song that sounded suspiciously close to the riffs of Kenny G and Earth Wind And Fire's cover of Outkast's "Way You Move", popular at the time.
A Brazilian TV ad for Chery (a Chinese car manufacturer) has a song that sounds very similar to Peter Bjorn and John's "Young Folks", with the exact same drumbeat, but a different (but just similar enough) whistling melody. Never mind that an earlier car ad in Brazil had used the same song (albeit it was The Kooks' version).
A Time-Warner Cable ad selling a football package used a song that is quite similar to the NFL Films song known as "Round Up".