Do you know a website that implements a menubar at an angle between 0 and 90 degrees?

You should look at here

http://www.useragentman.com/blog/2010/03/09/cross-browser-css-transforms-even-in-ie/

or use like this

Update:

/* for firefox, safari, chrome, etc. */
-webkit-transform: rotate(-90deg);
-moz-transform: rotate(-90deg);
/* for ie */
filter: progid:DXImageTransform.Microsoft.BasicImage(rotation=3);
/* for opera */
-o-transform: rotate(-90deg);

see example http://jsbin.com/ajoqe/4

Update: 2

or use this extension

Universal CSS Transforms: Rotate(free extension) : http://www.dmxzone.com/go?17422#Overview

* Pure CSS based - No Flash required! 
* Fully cross browser compatible - The Universal CSS Transforms: Rotate

is a jQuery based, build only with HTML & CSS - no Flash what so ever! It even supports IE6 next to the other major browsers. * Search engine friendly – The Universal CSS Transforms: Rotate is pure HTML and CSS based and generates search engine friendly HTML code that can be nicely indexed by all search engines and web spiders.

update 3

or use this http://code.google.com/p/jquery-rotate/

Two JavaScript image handling implementations are supported:

1. using DXImageTransform filter for Microsoft Internet Explorer
2. using Canvas object for other browsers

The library has been tested with:

* Mozilla FireFox 2.0.0.2
* Internet Explorer 7.0
* Opera 9.1 (note Opera 8 is not supported)

Just because the image is at an angle doesn't mean the hit area (the link) needs to be at an angle...

Stuff like this is usually done using images and - if you want a non-rectangular click area - using image maps. It's not very difficult to do but a lot more work than a straightforward ul.

The CSS rotate capabilities are not wide-spread enough yet to achieve a consistent effect in all browsers, but it might be an option to build a cross-browser menu that looks okay unrotated, and great when rotated.