Ask HN: Programming Project Ideas?

Do you have any project ideas for medium (say a few kloc) programs that would be good for learning? I want to work on something for a few weeks to just focus on programming (i.e. not trying to build a product). Examples I've heard previously are a BitTorrent client or IRC client, but something not networking focused would be cool too. I'm especially looking to work on something where I can learn a lot from the design (code, not UI) phase. Thanks!