MathSciNet a subscription only service (but available on most university campuses) provides BIBTEX entries for the entire mathematical literature.

A nice aspect of their interface is a "clipboard", to which you can save articles, then ask for the BIBTEX for everything on your clipboard all at once.

Mathematicians might also be interested in the shell scripts I wrote that automatically look up BIBTEX entries from MathSciNet, based on missing references in your .aux files.

For books I usually use a site, where it is possible to get Bibtex citations from Amazon.com articles. This is very good for books, and some inproceedings and incollections might be found here as well.

Amazon: http://lead.to/amazon/en/

As a Software Engineer I quite often have to deal with technical papers from ACM or IEEE. Both their catalogs provide Bibtex export capabilities.

ACM: http://portal.acm.org

IEEE: http://ieeexplore.ieee.org

If you use Firefox and the wonderful extension Zotero, it can generally parse citation information from any webpage, and export a BibTeX-style citation from it.

Mendeley allows you to sync your collection with a bibtex file.

Since it hasn't appeared in the other answers, Google Scholar also allows you to download a BibTeX citation for each of its search results. You have to enable the feature from the Preferences page, though.

I maintain my Bibtex database manually (using JabRef as a GUI).

In particular with conference proceedings, it's not possible to find everything that you need in one service, and if you copy & paste information from different services, your bibliography won't be internally consistent. And even if you copy Bibtex entries directly from the publisher's site (e.g., ACM and IEEE services mentioned in other answers), you will get a lot of garbage. Details such as accented characters in authors' names, math in titles, etc., are very often wrong.

MathSciNet is one of the very few sites that I actually trust so much that I usually copy & paste Bibtex entries almost verbatim. DBLP is useful but I nevertheless double-check the information that I get from it.

It's a lot of work initially, but as your Bibtex database grows, you will be able to reuse more and more entries in your new articles - especially as you don't need to check your bibliography again when you are preparing the final versions of your papers. Using Bibtex macros and/or cross-references helps a lot with the manual work.

DBLP has good BibTeX entries for much of computer science. Their coverage is not comprehensive, and they have some awkward gaps pre-2000, but it is one of my favourite sources.

Edit, five years later: DBLP is still a mainstay of my bibliography needs, and now covers 3 million papers, including nearly all papers in the bit of theoretical computer science I am interested in, as well as much of discrete mathematics. It now also includes citations for ECCC and arXiv papers (although these really should not be of type article with the repository as the journal).

Another possibility is doi2bib.org which generates BibTeX citations from digital object identifiers (DOI), available for most (recent) publications.

If, through some peculiar combination of circumstances, you find yourself citing a Wikipedia article, the "cite this page" item under the "toolbox" in the left-hand sidebar provides BibTeX information. For example, to cite what is now the current version of the Isaac Newton article, one clicks "cite this page" and receives

 @misc{ wiki:xxx,

   author = "Wikipedia",

   title = "Isaac Newton --- Wikipedia{,} The Free Encyclopedia",

   year = "2010",

   url = "url{http://en.wikipedia.org/w/index.php?title=Isaac_Newton&oldid=374986805}",

   note = "[Online; accessed 26-July-2010]"

 }

The URL in this block includes a reference to the specific version number, so even if the article is changed later, the version being referenced can be retrieved.

I use http://www.citeulike.org/ Briliant cite with loads of references and it is a bit of "social" referencing. You can create, import, export collections as bibtex and manage all of them online. It is pre-populated with loads of sources (I personally care only about IEEE which they pre-import via links.)

With this website the reference entry are by far the largest I saw including multiple web-view & purches links, full abstracts and customisable keys. Stopped writting bibtex files by-hand long time ago.

A few people have already mentioned MathSciNet (and noted that it is subscription only). The same BibTeX data are freely available from the AMS via MRef.

http://www.bibtexsearch.com/ offers a simple search interface to millions of BibTeX records.

The Spires database of high energy particle and astrophysics papers can display its results in Bibtex format.

They also have some tools to help update bibliography lists.

For computer science the Collection of Computer Science Bibliographies is very useful.

Slightly orthogonal to the actual question, but hopefully useful to anyone interested in the answers to it, there are libraries that convert from one format to another and these can often be more useful than finding a website that exports them in exactly the right format.

For example, for PHP then bibliophile has a library for converting to and from BibTeX. This is used by programs such as refbase for exporting references in BibTeX format.

Indeed, rather than a piecemeal approach, I would recommend using a reference program which can import and export to several formats (including BibTeX). Some have been mentioned in the answers above already.

I use BibSonomy to find and share Bibtex records. You may not find everything there, but with more and more users this should improve in time.

Unfortunately, the largest source of medical, biological and bioinformatical papers, PubMed, doesn’t offer a BibTeX export (yet?).

As a workaround, there is the service TeXMed that transforms the numeric PubMed identifiers to BibTeX citations. It’s not very usable but still it’s better than nothing.

Many scientific journals have readily available bibtex references available on their websites.

Here is a random one I found on ACM for instance:
http://portal.acm.org/citation.cfm?id=1291536&jmp=cit&coll=ACM&dl=ACM&CFID=96100051&CFTOKEN=92185154#

Since it hasn't been mentioned yet: Nelson Beebe maintains an extensive database of references for mathematics and computer science.

Recently I found a website which gives bibtex by isbn numbers.

ottobib

Use this site:

PubTransformer

to transform any pubmed paper into bibtex and other formats such as: ADS, EndNote, ISI used by the Web of Knowledge, RIS, MEDLINE, Microsoft's Word 2007 XML.

In my experience Jabref doesn't handle certain characters well, and there's always quite a bit of manual fixing involved when someone uses it to translate some other format to bib format. Endnote comes to mind.

For the past few months I have been using bibtex-search which is a great little node.js program written or maintained by ekmartin that you use in the CLI/terminal. All that you need to do is open up the terminal and enter bibtex-search -source google followed by the title of work being sought and the last name of the author. It will return the bibtex source and copy this to your clipboard for you! so that all you need to do is paste it into your bib file. I use jabref with Docear so I do not necessarily paste the clipboard contents into my bib file; but I could if I wanted to.

Installation

Assuming that you already have node.js installed...

$ npm install --global bibtex-search

Usage

$ bibtex-search <query>



Options:

  --source, -s Where to find papers from (default: acm) - valid options: [acm, ieee, google]



Examples:

  $ bibtex-search bayou

  $ bibtex-search --source google zaharia spark

Qiqqa.com helps you quickly and semiautomatically associate bibtex with your PDFs. You can then export a .bib file of all your references for use with latex/bibtex.