Thomson Reuters publishes impact factors on its service Web of Knowledge (http://apps.webofknowledge.com), but it's not free - see if your institution has a subscription. However, when publishers post the IF scores on their websites, they normally refer to Thomson Reuters' scores. I'd say that's good enough for most purposes, unless you have a reason to think they're lying (as in the case low-quality or completely new journals).
You must have seen that the journals appearing in such list are mostly new and paid journals. Original one which is accepted widely is JCR Thomson Reuters list.
There are many type of impact factors and journal indexes.
The most general is Google Scholar related, they took everything and calculate the indexes. Google Scholar nowadays has generally been accepted as one of the general measurement for personal h-index. I can see in many slide presentation about researches and publications, Professors start to use Google Scholar index as one of the tools to advise other authors.
The most highest quality is Thomson Reuters, they only count good quality journal according to their standards.
There are many other indexes that measure the journal "quality".
For beginners (early carrier of research), I propose just refer to Scopus index journal, that is minimum and basic requirement for starting point to good journal.
Here is some of the sample of indexing related to journal.
Hi, Thomson Reuters publishes impact factors on its service Web of Knowledge. That is the official source to know the impact factor. But, it is not a free service. However, you may know the latest IF in the journal homepage itself. Normally, it will be correct.
Now a days over enthusiastic journal publishers are misinterpreting index copernicus score as a journal impact. But please note that Journal Citation Report managed by Thompson Reuters, releases impact factor each year that can be checked