The first thing that came to mind upon reading this starting post are the forums over on AlternateHistory.com. This is the playground of hundreds of hobby-historians and history-buffs in general, where they keep thinking up new and exciting (and sometimes very absurd) ways of changing this or that detail. Sometimes they end up creating a story (or timeline as they're calling it over there) from the idea.
A word of warning, it's a very different environment than SOL and the content is being published in a forum. While this allows for a lot more reader participation and discussion as the story/timeline is actively being written, it is also more difficult to follow and the writing is considerably less polished in general.
My current favorite over there is But I Don't Speak German!, a story with sci-fi elements (time-travel, body displacement) wherein a college-age Irish guy ends up being placed in the body of a German WWI veteran in the year 1920. It's not too difficult to imagine what he'll want to change.
Here's one more link for you, the Alternate History Wiki, Timelines and Scenarios. I don't know if the lists are entirely up-to-date, but it should be a good start if you want to give the forum a try. I personally think that finding worthy timelines with engaging storytelling and a good plot idea can be a bit difficult, mostly I browse through the more popular new threads and sometimes find a nugget of gold.