I've got to say that even though it's got its idiosyncracies (no doubt stemming from stupid design decisions made for office suites elsewhere long ago that OpenOffice copied and never questioned), LibreOffice isn't actually all that bad... well, the Writer component, anyway.
I needed to write a document earlier, one that'll still have to be reviewed, and in order to clearly mark it as a draft, I wanted to overlay a big, light gray "DRAFT" (rotated 45°) on each page. With a bit of fiddling, I found out how to do this, too, so in case you're ever in the same situation:
- Create a drawing. You'll have to do this because ordinary text can only be rotated in 90° steps.
- Alas, the "Insert" menu has nothing of that sort: you cannot insert drawings that way. So instead, select View→Toolbars→Drawing to show the Drawing toolbar.
- Click the Text ("T") icon, and DRAG it across your document to create a text drawing.
- Enter the desired text and style it to taste (e.g. Helvetica, 144pt).
- If you don't have a "Text Color" button in your Text toolbar...
- Click on the Drop-Down menu indicator at the right end of the Text toolbar.
- Under "Visible Buttons", click "Text Color".
- Now change the text color, e.g. to Gray 10% or Gray 20%. (Light Gray is a little darker than the latter).
- If you want a color that is not in the palette...
- Go to Tools→Options→LibreOffice→Colors.
- Enter a name for your color and put in the desired RGB values (e.g. 220,220,220) and click "Add".
- The new color will now be in the palette.
- Click outside the drawing to leave it.
- Right-click on the drawing's border and select "Text...".
- Make sure both "Fit width to text" and "Fit height to text" are checked.
- Right-click the drawing's border again and select "Position and Size...".
- Anchor the drawing to the page, and set the horizontal and vertical position to "Center" (to "Entire Page").
- Switch to the "Rotation" tab and select the desired rotation (e.g. 45°).
- Right-click the drawing's border yet again and select Arrange→To Background.
EDIT, 2017-02-05: LibreOffice 5.3 doesn't have the "Text..." context menu item anymore; the options to fit the text box's size to its content are found in Format→Text Box and Shape→Text Attributes now.
EDIT, 2017-07-29: LibreOffice 5.4 finally has native support for document watermarks. Select Format→Watermark, enter your desired text and choose your desired font, angle, transparency text color. The defaults (Liberations Sans, 45°, 50% transparency, Light Gray) work very well.
You cannot (yet?) change the font size, so if you want to do that or otherwise get fancy with your watermark, the above method still works. Just remember to manually copy your watermark to each page; there is still (to my knowledge) no option to have a text box repeated automatically on each page.
Also see this blog entry
for more information.
EDIT, 2019-06-19: needless to say, this keeps breaking (changing) with every new LibreOffice release, and the solution keeps on eluding me. LibreOffice is terminally broken IMO; I recommend using Microsoft Word or so instead (for those few things that TeX can't easily do anyway). Yes, vendor lock-in sucks, but LibreOffice is broken beyond repair, and after all the years it's had to mature I've given up hope that it'll ever approach usability. (A fate sadly shared by a lot of Free Software.)