Steven Leckart for Cool Tools writes:

An avid scrapbooker in our family vehemently recommended these acid-free pens about five years ago. Ever since, we’ve used them to label all of our albums and prints, both new and old. If you’re going to take enough care to use an acid-free album or you plan to send your photos overseas for scanning, then you need one of these; otherwise, in time, the acid from, say, a standard ballpoint will cause degradation (yellowing, brittleness, etc.). The fine tip allows for small, really crisp penmanship (we use the 05 model which produces a .45mm line). They’re advertised as fade proof, non-bleeding and waterproof. I can attest to the first two (we take great care to keep our stuff dry *knock wood*). One caveat: to avoid smudges, let the ink sit about 10-15 seconds before putting the photo back into an album.

Zig acid free pens, available from Dick Blick and Amazon. Link.