Let's not rely on a server hosted on someone's PC here... Proboards might be a good option of last resort, but do we really want ads all over our boards? (or do we already have them? Wouldn't know, I use AdBlock...)
I probably already mentioned this, but NearlyFreeSpeech is a great low-cost host. I used to have several sites with them, with varying levels of activity, and I don't think it ever cost me more than $2-$3 per month (not including domain registration, but they have better-than-usual prices for that too). Though it has been a long time since I used them. They work on a "pay for what you use" basis rather than the usual package deals "X amount of storage space, Y amount of bandwidth per month, for fixed rate $Z" - and almost always work out cheaper.