Here is a good Economist article about low-skilled immigrant labor.  It links out to this Washington Post article, which is also good.  They discuss the "get in line" comment we hear from so many politicians these days with regards to immigrants coming here legally.  But "there is no line, not even a relevant visa category, for millions of immigrants."  Here's an excerpt from The Economist article:

Low-skill, undocumented immigrants can't "get in line", instead of entering and working in America illegally, because there is no line. As the Post notes, Republicans aren't alone in repeating empty "get in line" rhetoric; the president does it too.

…American demand for low-skilled workers remains. So they will come. And they will come illegally, because they must. It's very important to note that workers from south of the border act more as complements than substitutes for similar American workers.

That last link is great too and discusses how immigrants don't really steal American jobs.  Go check it out! 

(H/T to Paul Hsieh)