We are looking at doing a startup website that will hopefully receive lots of traffic.
I am interested to know what databases people use for their startups.
Obviously MySQL or other open source ones are the cheapest options, but would these hold up a serious site like Digg, Facebook, etc? Or would it be better starting off with something like Oracle?
These answers are all important, but it is possible to do a good job or a bad job with any of those languages. Ultimately the choice of platform, languages, database, etc. are not as important as how you put the pieces together.
You might also what other successful sites are using, particularly those with data structures similar to what you are planning. I believe Facebook and Wikipedia both use PHP and MySQL. FlightAware (which does a really super job with lots of real-time data) uses Postgres.
While it is, I'm sure, possible to design a very effective site with almost any combination but Windows/.Net/MSSQL seems like a particularly poor choice. There are only a few big data-driven sites using that technology and most of them aren't technically very good. Myspace is one example - (I think they use MSSQL) - but they have horrible response times and frequent hiccups. Ancestry.com is another big .Net site but they have a lot of problems. Pages frequently hang while loading, response not quick at all and their UI is very awkward. Several of the big airline sites run .Net but they aren't very good either. Big companies whose primary business isn't IT seem to go with .Net a lot and it shows in the quality of their sites - but that's not so bad - it creates a lot of opportunities for people like us. While I'm sure it is POSSIBLE to do good work on the Microsoft platform it seems very, very unlikely.
My personal choice after having worked, at one time or another, with all of the options discussed here is very clearly LAMP - Linux, Apache, MySQL, Perl|PHP|Python but I wouldn't object to substituting BSD, Lighttpd or Postgres.
As someone around here said recently, "...anyone proposing to run Windows on servers should be prepared to explain what they know about servers that Google, Yahoo, and Amazon don't."