Sources for BibTeX entries Does anyone know of a good place to find already-done BibTeX entries for standard books in advanced math?  Or is this impossible because the citation should include items specific to your copy?  (I am seeing the latter as potentially problematic because the only date I can find in my copy of Hartshorne is 2006, whereas the citations I can find all put the publication date at 1977.)
 A: I prefer the Zentralblatt database because it provides bibtex entries with DOIs.
A: I also use JabRef. In addition, there is an easy to use JabRef plugin for the Union Catalogue (GVK) of several German federal states participating in the so-called Common Library Network (GBV).
The GVK fetching is explained at http://help.jabref.org/en/GVK.
A: Another way is to use google with option "filetype:bib". Something like this.
A: If you use a mac, BibDesk is fantastic: among other really nice features, it lets you find your book/article/etc on your choice of free sites (ACM, arXiv, CiteULike, Google Scholar, HubMed, SPIRES) or subscription sites (IEEE Xplore, MathSciNet, Project Euclid, Zentralblatt Math) and then once you've found the item, it takes one click to import the citation into the database. The database can also store electronic copies of articles (if available) referenced to the citation.
So for example, I would open BibDesk, click on the icon that says Web, click on "MathSciNet". Within the program I see the MathSciNet page (assuming I'm at work where I have access). Type in the search terms Hartshorne and Geometry, and up comes 8 citations I could import. One of them is Algebraic Geometry from 1977, so I would click on the button that says "import". BibDesk does all the other work.
While writing a paper, just drag and drop the citations onto your LaTeX document to embed \cite{blah} with the appropriate cite key (at least if you're using TeXShop).
When you're ready to stick a bibliography into your paper, select the relevant articles in the BibDesk database and export them into a BibTeX file. It's super easy.
A: Since it's not been mentioned, I'll chime in with Mendeley, which provides BibTeX citations for papers on the site (minus the linebreaks, unfortunately). They also have a freeware reference manager.
A: About 10 years ago I wrote a perl script called "bibweb" which uses MathSciNet to produce bibtex entries for references.  If you have access to MathSciNet and you run
bibweb -c 'hartshorne;algebraic-geometry'

then one of its answers is
@book {MR0463157,
    AUTHOR = {Hartshorne, Robin},
     TITLE = {Algebraic geometry},
      NOTE = {Graduate Texts in Mathematics, No. 52},
 PUBLISHER = {Springer-Verlag},
   ADDRESS = {New York},
      YEAR = {1977},
     PAGES = {xvi+496},
      ISBN = {0-387-90244-9},
   MRCLASS = {14-01},
  MRNUMBER = {MR0463157 (57 \#3116)},
MRREVIEWER = {Robert Speiser},
}

It's not perfect, but it works for me. Download it at https://github.com/jhpalmieri/bibweb.  It's free, so satisfaction guaranteed or double your money back.
A: I recommend using the AMS website MREF, located here.
EDIT : Another remark about your question.  Don't worry too much about getting things like the printing date for a book correct (it changes every time they make a new printing run).  Just make sure that you have the author, title, and edition in some standard format (and for papers, the journal, date, and page number).  Every journal will reformat things into their house style and verify that your bibliographic entries are correct.  Make those copy-editors work for their money!
A: Try http://www.ottobib.com/. It is a ISBN to Bibtex (or other formats) converter.
A: I wrote a script using AutoHotKey that allows me to highlight the title of a reference (e.g., in a PDF article). It then looks up Google Scholar based on that title using Google Scholar's "allintitle:" key word.
I have Google Scholar set to export BibTeX.
I can then easily drag and drop the BibTeX into JabRef.
I posted the details on my blog:
http://jeromyanglim.blogspot.com/2010/04/how-to-get-quick-access-to-full-text.html
A: I have had problems with this earlier, and I agree with Olivier, that Zentralblatt's database is pretty good. I've even created a simple program which loads them easily to JabRef or any BibTex files. It works properly, try it out if you are interested. You can find it at https://sites.google.com/site/jabrefzentralblatt. No install or anything needed.
A: Google books also gives bibtex references and at least once I found a reference there for a math book not on Mathscinet for some reason.  To get the bibtex you go to the bottom of the "about this book" page and click on "Export citation: Bibtex" (you can also get Endnote and Refman references... whatever those are).
A: Google Scholar will produce BibTeX entries if you turn on Show links to import to BibTeX in Scholar search preferences.
A: Check out http://www.bibtexsearch.com - it has several million bibtex records.
A: MR Lookup is also a good tool and seems to be free.
A: There's also Lead2Amazon which can export in a variety of formats.
A: The best thing about using MathSciNet for bibtex entries is that it is standard and universally accepted. No need to reinvent a bicycle.
A: The Courant Centre has a useful webpage which produces nice BibTeX entries for arXiv preprints, if you input a paper ID or an author ID.
Random fact: The K-theory archive has a loooong BibTeX file on the K-theory literature available for download.
A: Zotero is a nice plug-in for firefox that produces a database of your favorite publications. Every time you are on a website like arxiv, math-sci net or the homepage of some journal, it offers you to save the publication into your database. It gathers all the relevant information automatically from the page, and it does that amazingly well. Once you've got all your publications in your database, just hit the export to bibtex button and you're done. 
A: I use doi2bib a lot - journals usually provide doi, but the bib entry (if present) is hidden or buried in some file. I rather just copy it from a field.
Same with isbn2bib, very useful for books.
A: Currently I get very good quality bibtex entries at Crossref, hope it will last
