I really like Tenacious D. I think what I hate about them is that they don't take themselves seriously. Some of their songs are really good, despite being somewhat of a joke... For example, from their first album:
Or this, the final showdown (from The Pick of Destiny):
* Tribute contains only one quick swear. Beezeboss is full of them.
If you heard both songs and watched the movie:
[spoiler]The funny thing is that Tribute talks about a time when they beat a demon by playing "the best song in the world," that they can't remember. In Beelzeboss, they do beat a demon with what they later say is the best song in the world. However, when they try to play it again, they forgot it. So we can wonder if Tribute is about Beelzeboss, and also if Tribute was the song they wrote with the Devil's horn.[/spoiler]
Their third album, Rise of the Phoenix, is supposed to be released this year. If you want more songs, Kyle Gass has another band called Trainwreck (KG is seen using a Trainwreck T-shirt in the movie and many live shows).
EDIT: since we're on the subject, I want to talk more about it.
The first video I posted, Tribute, features a cameo from Ben Stiller. The second one, Beelzeboss, features Dave Grohl, from Nirvana and Foo Fighters, as the Devil. Grohl is a long-time friend of Jack Black.
Since we were in the subject of Dio a moment ago... Tenacious D has always been very supportive of their idols, and has appeared with them many times. Another song from the film The Pick of Destiny features Meat Loaf as JB's father, Ronnie James Dio as himself and shows JB's room filled with rock band posters, such as Queen, Black Sabbath, The Who and AC/DC. It ends right where Classico, posted by InoFan above, starts. It's the movie's intro, so it doesn't contain spoilers, but has quite a bit of bad words. Also a very fun song.
Dio, who is Black's biggest idol, also appeared in this short Tenacious D video:
Tenacious D, the band's first album, contained a song dedicated to Dio (the song is called Dio), though I'm not a big fan of the song.
I love the album's first song, Kielbasa:
Listen, enjoy, and have a nice day.