Vocaroo is a new, free, and very easy way for students to record audio messages. It would be an excellent way for English Language Learners to practice speaking.
One great feature is that not only can you email the link, but you can also embed your audio message in a blog or website.
Daft Doggy Voice Recording is still the easiest way for students to record and send these kinds of messages (Vocaroo, though very easy, is still a little more complicated).
Links to both sites and other similar ones, are on my website under Student Audio Messages.
Thanks to the Make Use Of blog for the tip.