Probably the closest analogy would be seeding on bittorrent. No harm to you, beyond perhaps reduced network speed, but significant benefit to everyone else.
Unfortunately, in many countries[1] you'll be prosecuted as an accessory if someone commits a crime via your connection, unless you set it up in such a way that you're legally an ISP. This is obviously a lot of trouble, so you're better off not bothering.
[1] there have been a number of cases in Germany, for example
