Descriptions

Slantastic (http://meyerweb.com/eric/css/edge/slantastic/demo.html) supports old browsers. For CSS3-specific, try CSS polygons: http://alastairc.ac/2007/02/dissecting-css-polygons/.

Code

The HTML:

<div class="cornered"></div>
<div class="main">Hello</div>

The CSS:

.cornered {
    width: 160px;
    height: 0px;
    border-bottom: 40px solid red;
    border-right: 40px solid white;
}
.main {
    width: 200px;
    height: 200px;
    background-color: red;
}

The result: http://jsfiddle.net/mdQzH/

Alternative Code

To use transparent borders and text in the border section… The HTML:

<div class="outer">
<div class="cornered">It's possible to put text up here, too
    but if you want it to follow the slant you have to stack
    several of these.</div>
<div class="main">Hello hello hello hello
hello hello hello hello hello</div>
</div>

The CSS:

.outer {
    background-color: #ccffff;
    padding: 10px;
    font-size: x-small;
}
.cornered {
    width: 176px;
    height: 0px;
    border-bottom: 40px solid red;
    border-left: 40px solid transparent;
}
.main {
    width: 200px;
    height: 200px;
    background-color: red;
    padding: 0 8px;
}

The result: http://jsfiddle.net/76EUw/2