I'm working with a quite a big XML file (~20-40 MB) and I'm using Notepad++.
Is there any better application to view and analyze such a big XML files?
EmEditor is a text editor for Windows that allows you to view and edit extremely large files. There's also an Outline plug in that will let you see the structure of the XML document.
LibXML/LibXSLT is a good free tool (command line): http://xmlsoft.org/
MyEclipse is an addon for Eclipse, but costs money.
As mentioned before, XMLSpy is also very good.
XMLSpy is an excellent tool for analyzing XML files (even quite big ones). However, it is not free.
Because XMLSpy and Oxygen are already mentioned. To view and validate really huge XML files you can also use XML ValidatorBuddy which uses the Xerces SAX parser and has a more reasonable price. Of course you can edit XML and other text files also.