I'm thinking about getting a Samsung Galaxy S phone, I want to know a bit more about its capabilities as an ebook reader
Does it have any built in applications for reading? And if so which formats does it support?
Android 2.1 doesn't have any preloaded ebook readers but specific phones may have something preloaded. The pre-loaded apps depend on the carrier, but regardless you can download different ebook readers from the Android App Market for free. Unfortunately none of the current ebook readers are a complete solution as they all have major limitations. Here are the main players:
Free & Non-proprietary (free app & free books)
Aldiko: Supports ePubs with and without Adobe DRM. There are thousands of these books available; Aldiko also has an included store selling commercial, DRM-protected books. Great user interface. Aldiko also has a paid app that has more features.
There are a bunch more that do the open source books in the app market but none hold a candle to Aldiko.
Proprietary (these are all free apps but in most cases you have to buy the books)
eReader: Only reads PDB files and you have to sideload the app. There area several book stores that sell books in this format including ereader.com and fictionwise but these books are usually twice as much as Kindle or Nook books. Barnes & Noble used to sell PDB ebooks but swicthed to a DRM'd ePub. This has got a great (powerful and easy) UI and used to be my favorite when I could get books from B&N but now it is useless to me.
Nook: Reads open ePubs and DRM'd ePubs you buy from Barnes & Noble. The UI needs a lot of work compared to eReader and Aldiko. *I just abandoned this for the Kindle app.
Kindle: (this one comes pre-loaded on the T-Mobile Galaxy S) Reads DRM'd (.azw) ebooks you buy from Amazon. *This is now my ereader of choice.
Kobo: Reads ebooks from kobo.com. I haven't bothered with this since the Nook came out. It used to be able to read Borders.com books but I don't know anymore. The old version of the app didn't allow for offline reading but they might have fixed that.
Most of the major proprietary readers (Nook, Kindle & Kobo) all have about the same selection of new release books at the same price (about $12.99 per book). I expect these apps to get a lot better in the future it's just a very frustrating that I used to be able to use a much more robust and user friendly app (eReader) with new B&N books, but now I have had to downgrade to the Nook.
But to answer you question (got off on a tangent there, sorry): If you get the Galaxy S from T-Mobile you'll have Kindle pre-loaded, but even if nothing's pre-loaded you can download all or any of the above mentioned apps for free from the Android Market (except eReader which you have to sideload).
Update: Amazon just recently updated their Kindle App with a few key features (that I came to love in eReader) that make it a Nook killer in my book: 1. Ability to highlight text and make notes 2. Ability to long click on a word and look it up at dictionary.com or wikipedia. I'm switching to the Kindle App now although I'm annoyed that I just bought two Nook books.
In the US, at least, most editions of the Galaxy S are shipped with Quick Office, a pretty functional document viewer that includes Word & PDF support. It's basic, but works.
Beyond that, quite a few 3rd party apps are available on the market, including Adobe Reader, Amazon Kindle, Barnes & Noble Nook, and a handful of readers supporting non DRMed formats like TXT and epub.
I prefer FbReader but none of the android readers are as good as Stanza for iPhone. I find them all quite disappointing:(.
My TMO Vibrant came with the Kindle app, but as Saiboogu said, they're all downloadable, too.
I am using Laputa reader and I am very much satisfied with same.
My Samsung Galaxy S came with the Aldiko e-reader pre-installed. Apparently, this is pre-installed by Samsung, not my carrier. It is specified in the official Galaxy S website in the user guide, under tools.