Are there any sites that are similar to Wolfram Alpha, or is it currently unique?
Wolfram Alpha is the only of it's kind, when it comes to understanding chemical reactions, mathematical formulas, comparing objects etc.
The search engine itself is not unique and most of the data you could get else where. But the way that it tries to understand your statement and group relevant info with graphs etc. only showing 1 result is not anything I've seen or heard about anywhere else.