A good example of this is his collaboration with Calle 13. Called Cumbia Sobre El Rio, it might be his most famous song. At least it is the one I hear most often in a bar or club late at night when more eclectic music gets played, before the reggaeton comes out.
Calle 13 is a hip-hop group from Puerto Rico. It's cool that these styles, hip-hop and cumbia, can fit so nicely together. Another big song from Calle 13 is also a collaboration, No Hay Nadie Como Tu, this time with Cafe Tacuba.
Now, old-school Celso Pina was much more traditional and Mexican sounding. But somewhere along the way he got a sick group and modernized his sound. Other favorites of mine are El Porro Magangulano, a funky tune with lots of horns that sounds almost like afrobeat, and Hasta Siempre Comandante, a cover of a Cuban song about Che Guevara.
If you would like to know what more traditional cumbia sounds like, look for a band called Sonora Santanera. They wrote some of the biggest hits that you always hear in the dance clubs of Latin America. But for coolness and originality, go for Celso Pina.