OpenStreetMap
The OSM wiki seems to indicate that no one has yet come up with a good scheme for adding stop signs to OSM. There is an existing highway=stop tag that can be applied to nodes, but it provides no way to specify which ways connected to that node actually have a stop sign.
So, I suggested a new mechanism for tagging stop signs using relations that should be flexible enough to handle any kind of situation involving stop signs that I've been able to think of. I have submitted a formal proposal yet, but I did add a description to the highway=stop discussion page.
In the meantime, I tried out the new scheme by tagging all the stop signs on my bike route to work along S 7th St., N 7th St. and N. 4th St. near downtown San Jose.
This route doesn't have anything too exotic, but it does have a few things beyond the garden variety four way stop.
This tagging scheme does generate a *lot* of relations, and road with a lot of stop signs will have a lot of relations following this tagging scheme.
If anyone is curious to see what I did, you can examine the map area indicated in this diary entry.
Discussion
Comment from andrewpmk on 3 December 2008 at 02:04
I think it would be best to apply the relation to two nodes, not a node and a way. Please see my alternative proposal for details.
Comment from Icycle on 3 December 2008 at 02:34
For what it's worth, I think both proposals would work and would resolve the deficiencies of the highway=stop tag, but I personally find it more intuitive to specify the stop sign via way and a node, since the way and the node node are natural participants in the junction at which the stop sign occurs, but if you specify the stop sign via two nodes, the "from" node could be quite some distance from the intersection, and the stop sign could become undefined if that node gets deleted.