A page tab can work as a link to another webpage. The way to do that is publish a blank page with a link to that webpage in its title box. Html code model:
Officially you cannot have html code in title boxes (it either gets stripped out or shows up as plain text instead of producing the intended result). But when I experimented with the link code, I found it works – on one (still mysterious to me) condition: the link name must include a space. So a single word won’t do; two or more words, or one word plus a space, will.
Note 1: You can’t make such a link open in a new browser window: the code for that gets stripped out.
Note 2: Linking to another webpage includes the dynamic pages of your blog; so the trick can also be used e.g. for creating a “Home” header tab in themes that don’t have one, or for turning a post category into a header tab.
Note 3: For a Home tab in particular, there’s a better alternative – (free of the problems explained below): add a new page, title it Home, save draft, change slug to index-php, click ok, publish. (Thanks to Elizabeth for sharing.)
Note 4: In wp.org blogs you can create a linking header tab by using this plugin. (Thanks to Waffles for pointing out.)
In some themes you need the Pages widget (so that your page tabs will show up in the sidebar or bottombar), while in other themes you automatically get page tabs in the header area (see here). In the first case, the page-as-extra-link trick works flawlessly, but you don’t really need it: you can add the link to your regular links, or put it in a text widget. The trick is more useful in the second case, i.e. when you want to have the link as a header tab.
But this trick is essentially a violation of what should be possible, and it doesn’t work properly with all themes. In some themes you get a perfectly normal tab, in others you get a minor defect you can live with, and in some you get very bad or downright useless results. In general, the defects have to do with the fact that this irregular page somehow registers as a double one: the actual page you have created, and the link proper.
|(1) Ok in:
Also ok in
Also ok in
Also ok in
(Also ok in
|(2) So-so* in:
(see details &
(3) Major problem* in:
(see details &
 Ok but not needed: the theme includes the option to add “Extra Links” (under Appearance > Theme Options).
 Thanks to Tzaddi for sharing the discovery.
Details for (2)
Chunk, Day Dream, DePo Masthead*, Freshy, Iceburgg, Neutra*, Pressrow, Spring Loaded – wider space before the tab, e.g.:
In most cases the extra space is a blank tab linking to the actual static page you have created (not visible unless you hover over it).
Duotone, Steira: wider space after the tab.
Contempt, iNove – visible blank tab before the tab:
[*] In the themes marked with the asterisk, the visible or invisible blank tab won’t show up if the page is the leftmost one (see here how to set page order).
Details for (3)
Bueno – tab lower (below blank tab):
Chaos Theory – tab lower:
Digg 3 Column – tab below blank tab, defective, title hidden by header image:
Enterprise – tab lower:
Garland – tab lower, extra arrow (plus background color problem):
Greyzed – tab lower (below blank tab):
Light – extra blank tab plus title offset to the right:
Misty Look – tab slightly below blank tab:
Monochrome – tab below header:
Notepad – tab lower (below blank tab):
Ocean Mist – nasty gap:
Regulus – tab below blank tab:
Rounded – nasty gap:
Sandbox 1.6.1 – tab lower.
Structure – tab below invisible blank tab:
Unsleepable – tab below blank tab, invisible (unless you hover over it):
Wu Wei – tab lower.