Where to publish helpful, but informal papers?

If you don't know where to publish something, but nonetheless it is of an appropriate standard and you consider it worth sharing, why not posting it to arXiv? You won't get prestige just for submitting, but:

• people can find it,
• you can easily point to it,
• it is timestamped.

Submit it to a conference that publishes a proceeding. Check the ACM SIGs. There are at least a couple that might work.