Publicly Available Datasets One of the common problems in data science is gathering data from various sources in a somehow cleaned (semi-structured) format and combining metrics from various sources for making a higher level analysis. Looking at the other people's effort, especially other questions on this site, it appears that many people in this field are doing somewhat repetitive work. For example analyzing tweets, facebook posts, Wikipedia articles etc. is a part of a lot of big data problems.

Some of these data sets are accessible using public APIs provided by the provider site, but usually, some valuable information or metrics are missing from these APIs and everyone has to do the same analyses again and again. For example, although clustering users may depend on different use cases and selection of features, but having a base clustering of Twitter/Facebook users can be useful in many Big Data applications, which is neither provided by the API nor available publicly in independent data sets.

Is there any index or publicly available data set hosting site containing valuable data sets that can be reused in solving other big data problems? I mean something like GitHub (or a group of sites/public datasets or at least a comprehensive listing) for the data science. If not, what are the reasons for not having such a platform for data science? The commercial value of data, need to frequently update data sets, ...? Can we not have an open-source model for sharing data sets devised for data scientists?
There is, in fact, a very reasonable list of publicly-available datasets, supported by different enterprises/sources. 

Some of them are below:


Public Datasets on Amazon WebServices;
Frequent Itemset Mining Implementation Repository;
UCI Machine Learning Repository;
KDnuggets -- a big list of lots of public repositories.


Now, two considerations on your question. First one, regarding policies of database sharing. From personal experience, there are some databases that can't be made publicly available, either for involving privacy restraints (as for some social network information) or for concerning government information (like health system databases).

Another point concerns the usage/application of the dataset. Although some bases can be reprocessed to suit the needs of the application, it would be great to have some nice organization of the datasets by purpose. The taxonomy should involve social graph analysis, itemset mining, classification, and lots of other research areas there may be.