I suggest you use Mendeley! The space you have as a free user is quite huge. All I do is to import only relevant publications into the software when ever am writing a report. I suggest you do same.
My personal preference was to find something which is least intrusive to the folder structure I have. Tried few known ones but ended up with nothing, except a good bibliography tool - Jabref. Its free, java (so portable) and writes to a .bib thats directly include-able in my LaTeX papers. It also provides facility to link physical doc/pdf to the bibliographical entry.
Yogesh: Mendeley allows BibTeX syncing which I've found extremely good.
Mendeley is my favourite reference management software, not just for reference management (which it does a very good job of) but also for organising research papers (including folders, tags, highlighting, notes, in-text search across your whole library).
We are big users of www.refworks.com at my university. It is a subscription product. A good alternative if your university already subscribes. The Proquest company which owns it is in the process of rolling out a new version
Mendeley has been great for me over years. Its free and allow online account. Modification of reference styles are also easy. Online assistance at Mendeley web is always available, just like in researchgate
Mendeley is still a solid choice, for all of the reasons others have mentioned here. It works well across multiple devices and has powerful tools to get your references right.
This week, a new reason has been added to that list. Mendeley has now been integrated into the collaborative text editor for researchers, SciFlow. It is now possible to cite collaboratively with your co-authors, by sharing references cited in documents with collaborators, which allows them to re-use those references where they see fit.
By logging into their Mendeley account and connecting their library to SciFlow, authors can simply search for and drag their references into the text to cite.
SciFlow's journal and university templates will then handle the formatting during the export process and create a publisher-compliant document for you.
If you would like to know more about what SciFlow can do, feel free to take a look at our website.
"Gaby Appleton, Managing Director of Mendeley, explains why the platform has been so successful:
"Mendeley Reference Manager has grown to serve over 8 million users from a standing start. The platform has really solved the problem of helping researchers organize their knowledge and making citations painless. The incredible growth of Mendeley is a result of this focus on meeting the needs of researchers."
Another emerging trend is the increased focus on the need to manage and share data, which is what prompted the recent launch of Mendeley Data, where researchers can search over 8 million datasets from domain-specific and cross-domain repositories and make their own datasets citable, enhancing the scope for reproducibility..."