Last year, I signed up to be a reviewer for Mathematical Reviews. For those of you not familiar with the publication, it is essentially a database of mathematics articles, with short "reviews" written by other mathematicians. These are not reviews in the ordinary sense of a book or movie review -- the reviewer usually doesn't venture an opinion of the work, and even more rarely expresses a negative one. Rather, the review summarizes the results and attempts to put them into context.
Since 1940, Math Reviews has provided an invaluable service for research mathematicians -- having a good summary of an article is important before one begins the arduous task of tracking down a journal article, and the sometimes more arduous task of reading it. This utility has only increased with the electronic form of the database.
My first review appeared this year, and is of the article "On congruence conditions for primality" in the journal Integers. I think you can access the review here if your institution subscribes, but for various uninteresting reasons, I can't confirm that.
Anyway, I'm particularly proud of my five sentences because I have been a Math Reviews reader for about twenty years, and it is nice to join the ranks of the reviewers. Also, I think I get a discount on my next year's AMS membership.