For science buffs and technology geeks who like to read free science papers,
arXiv.org is a well-known site. As of this writing, arXiv provides open access to 1,144,295 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics.
If you like to read those PDF papers on a tablet or mobile phone, then you should probably be interested in accessing the papers via a well-implemented OPDS catalog. Being able to do so is very convenient.
The free OPDS catalog for arXiv, which I've personally used for many months and would like to recommend, can be accessed at
http://arxiv.maplepop.com/catalog/
The OPDS catalog hierarchy perfectly mirrors that of the science subjects as listed on the arXiv.org website. Excellent!