LibraryWorkshop

Delicious

CiteULike

At the moment the database is dominated by biological and medical papers

If you posted a paper from a site that's not on the "supported" list, then it will appear in your library, but it won't be widely publicised on CiteULike

Sort seems to be by recent posting, but can also see how many others have tagged the article

Supported list:

2. Amazon
3. American Chem. Soc. Publications
4. American Geophysical Union
5. American Meteorological Society
6. Anthrosource
7. arXiv.org e-Print archive
8. Association for Computing Machinery (ACM) portal
9. BioMed Central
10. Blackwell Synergy
11. BMJ
12. CiteSeer
13. CSIRO Publishing
14. HighWire
15. IEEE Xplore
16. Ingenta
17. IngentaConnect
18. IoP Electronic Journals
19. JSTOR
20. MathSciNet (Experimental support)
21. MetaPress
22. NASA Astrophysics Data System
23. Nature
24. Physical Review Online Archive
25. PLoS
26. PLoS Biology
27. Project Muse
28. PubMed
29. PubMed Central
30. Science
31. ScienceDirect
32. SpringerLink
33. Usenix
34. Wiley InterScience

H2O

Basis of database appears to be reading lists from Princeton courses. I can't tell how they're sorted.