El Paso's sister city, Ciudad Juárez, is the fourth-largest city in Mexico with approximately three million residents. Together, the cities form the largest binational population in the world. Juárez is a regional manufacturing center, due to cheap, abundant labor, and companies such as General Motors and Sony have facilities in the city. Juárez is seedy in the same way as other border cities such as Nogales and Tijuana are, but it is more of a real Mexican city, not one that is built on tourism alone. Juárez's history and authenticity, in my opinion, make it an interesting stop for an afternoon, or even an entire day. (If you're headed specifically to Marfa or Big Bend, however, it probably isn't worth the diversion.)
Like El Paso, Juárez's modern history begins with Spanish conquistador Juan de Oñate crossing the Rio Grande in 1581. The oldest structure on the border, La Misíon de Nuestra Señora Guadalupe (Our Lady of Guadalupe Mission), was completed in 1668 and remains in remarkably good condition today. The city played important roles in the Mexican-American War and the Mexican Revolution, and was once frequented by Pancho Villa.
Today, the city's booming manufacturing industry is complemented by tourism, with many visitors crossing the border to take in the colorful outdoor markets, historic missions, and lively nightlife. Tourists often drive across the five bridges scattered around El Paso, park in downtown El Paso and walk across, or else take a taxi or a trolley tour. The bridges, aside from the "Free Bridge" (or Cordova Bridge) south of I-10 via U.S. 54, all charge nominal tolls, even to pedestrians, of 25¢ to $2. The most convenient points of entry are the two downtown bridges, at Stanton Street and Santa Fe Street. U.S. currency is welcome practically everywhere in Juárez.