How can you show/reveal hidden characters in NetBeans? In other editors, if this feature is turned on, a space might be shown as a small centered dot, and a tab as a right
This feature has finally been added and is available in NetBeans 6.10. Since this has not been released yet, you can try out 6.10 MileStone 1: http://bits.netbeans.org/netbeans/6.10/m1/. I look forward to the final release!
This feature was missing for a long time—a feature request was created on November 1999 and it was finally implemented on August 2010 (NetBeans 6.10). You can enable/disable it at "View → Show Non-printable Characters".
As of NetBeans 7.0.1, the definition for "non-printable" seems to include tabs and carriage returns but not regular white space char:

NetBeans 7.1.0 finally displays spaces. However, now it has a new bug where consecutive tabs (or tabs & spaces combinations) collapse into one tab:

NetBeans 7.4.0 fixes this:

There're yet a couple of issues:
The end of file is represented with the same symbol as line feeds, thus making it non-obvious to determine whether the file ends with EOL.
Hidden chars are displayed in the same colour as regular text.
These issues have not been addressed so far but they can be sort of mitigated with the Editor Whitespace third-party plugin, which:
View -> Show Non-Printable characters
in netbeans 7 you can enable display of spaces, tabs & line feeds like this:
in netbeans 7 you can enable to see non-printable characters:
in netbeans 6 only the spaces part works
Using Netbeans 8.0.1
View-> Show Non-Printable characters
Using Netbeans 7.3.1,
View -> Show Non-Printable characters
I can see white-spaces, tabs, and carriage returns.