A discourse marker used when you remember something or switch topics—equivalent to “that reminds me” or “now that you mention it.”
そういえば、昨日山田さんに会ったよ。
Speaking of which, I ran into Yamada yesterday.
そういえば宿題、出した？
By the way, did you turn in the homework?
そういえば、忘れてたけど来週は会社が休みだね。
Now that you mention it, I forgot—next week the company is closed.