I tried Zotero a while back after Ben suggested it (I'm sure it was you Ben) but couldn't get into it. However, Zotero has launched 'Zotero Everywhere' and now supports all the major browsers (except Opera)
As with the majority of online services it follows a freemium model i.e. it is free to store 100MB after which it costs $20 a year for 1Gb, $60 per year for 5Gb and so on.
I think I will give it another go. There is also another similar online app called Mendeley which is also worth checking out.
Zotero can be found at www.zotero.org
Mendeley can be found at www.mendeley.com
If you use either of these let us know whether you have found them useful or not.
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