The Internet has made everything easier -- and more complicated -- including plagiarism. In addition to being able to buy completed research papers online, students can also copy and paste directly from websites to their assignments (instead of copying reference books from the library).
But teachers have their tricks, too. Online tools (balancing out that tricky dichotomy between making life simple and complicated) are available for teachers to sleuth the suspects. Just copy and paste the copied-and-pasted text to find the source on Dustball, Article Checker, or Plagiarismchecker among many other sites.
Those sites are free, but for a fee, TurnItIn adds another twist to the crime scene: students can collaborate on reviewing each others' papers using the software. Interesting idea.