Is there any free XML editor/browser than can edit quite large (tens of megabytes) XML files, with built in XSLT support? Actually what I am looking for is Microsoft's XML Notepad 2007, but for OS X !

I haven't found anything good. There's some simple programs such as XSLPalette which can do the XSL transformation for you but the editor I've been using is TextMate. There's also a free XSL and XML plugin for TextMate that allows you to do validation against a schema and do the XSL transformation but I haven't found any good native OS X XML editors like there are on Windows. Actually I was somewhat considering creating one some day but that's going to be quite a bit of work.

I use Editix: http://www.editix.com/ (starting from 70$, there was a cheaper version a few weeks ago)
Also a free version: http://free.editix.com/

Nice soft: stable easy to use for edition and schema/DTD validation (I don't use more)

support also XPath and XSLT.

I tried the free version, but could not use xpaths; it seems you need the commercial version for that. I use XML/Notepad mainly in developing xpaths that will be embedded to code (XSL or otherwise), so an XML tool without xpath is no tool for me.

Have a look at BBEdit (http://www.barebones.com/products/bbedit/index.shtml), there's a demo available and there are many third party extensions (http://72.14.207.99/search?hl=en&q=BBedit+and+XML&btnG=Google+Search).

Maybe you'll like it but it's not free.