A place to find original papers I currently use scholar.google.com to find papers in cases like Sophus Lie's original papers on "Transformation Groups". Does anyone know of other places that collect original works like this, i.e. works by those such as Weierstrass, Riemann or Lie describing the ideas that they originated. scholar.google.com isn't "bad" per se, I'd just like to look up just those papers and not every paper on the same topic.
I have access to many journals so that isn't so much the problem as knowing where to look.
 A: I would also advise (especially if they were published in french journals, such as the articles by Elie Cartan, Frechet, Henri Cartan)
GALLICA
and 
NUMDAM
You can often download whole articles, depending on date and copyright.
Here you have an obituary for Sophus Lie in the Weekly Accounts of the French Science Academy in 1899 (CRAS 1899, p525).
3 important original articles from Sophus Lie, probably on GDZ for the first 2.
Sophus Lie, Über Complexe, insbesondere Linien- und Kugel-Complexe, mit Anwendung
auf die Theorie partieller Differentialgleichungen; Mathematische Annalen Vol 5, pp145-
256 (1872)
Sophus Lie, Untersuchungen über Transformationsgruppen. II; Archiv for Mathematik
og Naturvidenskab vol 10, pp353-413 (Kristiania 1886)
Sophus Lie, unter Mitwirkung von Friedrich Engel, Theorie der Transformationsgruppen III, 1895. Printed as a book I think. I have it as chapters in the Chelsea reprint.
A: Two resources that haven't been mentioned are Cornell University's digital collection of historical math monographs and the University of Michigan's historical mathematics collection.
A: On www.archive.org, you can find works by Felix Klein, Cayley, George Boole (and also by Mary Everest-Boole, btw - some also very nice.) Perhaps there are others there, too.
A: Besides JSTOR, NUMDAM, and the like there is a German digital math archive GDZ 
at gdz.sub.uni-goettingen.de/.   Some sources require access through a subscribing institution, by the way.   There is no central resource and may never be.   Copyright laws in many parts of the world pose a strong barrier to sharing material, though as some posts on Math Overflow illustrate there is a lot of unauthorized dissemination (as with recorded music).   Commercial ownership complicates the question of what information can be freely distributed.  This is currently a big issue in book publishing generally and mathematics in particular as E-books proliferate.   Even nonprofit publishers like AMS and Cambridge have hard choices to make.
ADDED: There is no shortage of Web sites to browse such as The Euler Archive at www.math.dartmouth.edu/~euler/ and many others accessed through the AMS e-math pages  e-math.ams.org/samplings/math-sites/math-sites.  Too many sites, too litle time.   Many are helpful only if you know exactly what you are looking for, which is however the problem most of us begin with.   
A: I have mentioned this link before. You can find links to almost everything that is freely and legally available on the web as far as mathematical publications are concerned.  
A: JSTOR is a very good digital archive of scholarly works.
A: Are you looking for just the papers, or also for commentary on them?  For the former, surely you can just consult the collected works of the people named.  For the latter, it seems likely that the collected works will also have at least some commentary on the work, perhaps with references to additional commentary.  You can also try looking up history of mathematics on mathscinet.  (The classification is 01.)  
For Sophus Lie in particular, try the book "The genesis of the abstract group concept", by Hans Wussing, and the detailed analysis of the history of Lie groups by Thomas Hawkins.  (Look on mathscinet for his long list of publications on this topic.)
A: Google books is useful in some cases. I found a couple of full view books about Sophus Lie's original papers including one on Transformation groups:


*

*Sophus Lie's 1880 transformation group paper
by Sophus Lie, Róbert Hermann

*Sophus Lie's 1884 Differential Invariant Paper
This is volume one is the series "Lie groups: History, frontiers and applications."
A: On www.archive.org they seem to have most of Weierstrass,
a lot of Lie, and all of Riemann.
A: Your university library, and interlibrary loan.
