From time to time it is useful to have a detailed look at the kind of errors that learners of English make. The International Corpus of Learner English from Université catholique de Louvain (Belgium) has an error-tagged corpus of written text produced by learners of English. More information here. Unfortunately, it is not freely available online, but the price is modest. You can also gain access to it by sending them your students' texts (after, I assume, receiving permission from your students.)
Erors can be amusing as well as edifying, such as this one. Anders Henrikson has been collecting students' mistakes for many years. Here's a collection that has been around since at least the early 1990s. He's also got a book of them.
Finally, last week the Language Loggers posted a link to a video of Taylor Mali changing all the usual typos into speakos.