OUTDATED, see IF 2017 for new data.
The Semantic Web journal just got an impact factor, and it's excellent indeed.
If you search the Journal Citation Reports system for all journals which have "Web" in their title, you find 7 of them. SWJ tops the list with 1.786, and the next best one is Springer's "World Wide Web - Internet and Web Information Systems" with 1.539.
The full list is as follows: