A very guilty pleasure, written and directed by Robert Rodriguez. As IMDB will tell you, its unusual origin was as a fake trailer in Grindhouse, a (tribute to|parody of) gory low-budget thrillers of past decades. The hero, Machete, is played by the spectacularly ugly Danny Trejo; if you saw any of the Spy Kids movies, he played the same guy, as "Uncle Machete". But nobody calls him Uncle here.
A brief opening scene establishes both Machete's bad-assery and his ultimate nemesis, the ruthless Mexican crimelord Torrez (Steven Seagal!) who deprives Machete of his friends, family, and nearly his life.
A few years later, up in the US, Machete is living an odd-job life as an illegal immigrant. But soon enough he gets (completely coincidentally) roped into a complex (completely unbelievable) conspiracy. A host of bad guys wants him dead. Fortunately, he has his eponymous weapon close at hand.
The plot, such as it is, is cartoonish and tendentiously hung on the issue of illegal immigration: the opponents are uniformly murderous, venal, sadistic, perverted, and all under the control of aforementioned criminal Torrez. (In addition to Seagal, there's Don Johnson, Jeff Fahey, and Robert De-frickin'-Niro, along with dozens of machete-fodder minions.) Fortunately, there are some good guys too: Cheech Marin, Jessica Alba, Michelle Rodriguez.
Also appearing: Lindsay Lohan's boobies. Probably the rest of her too, although I didn't notice.