Now in HTML5/CSS3 we have better solution for the problem. In my opinion this purely CSS solution is recommended:

table.fixed {table-layout:fixed; width:90px;}/*Setting the table width is important!*/
table.fixed td {overflow:hidden;}/*Hide text outside the cell.*/
table.fixed td:nth-of-type(1) {width:20px;}/*Setting the width of column 1.*/
table.fixed td:nth-of-type(2) {width:30px;}/*Setting the width of column 2.*/
table.fixed td:nth-of-type(3) {width:40px;}/*Setting the width of column 3.*/

<table class="fixed">
    <tr>
        <td>Veryverylongtext</td>
        <td>Actuallythistextismuchlongeeeeeer</td>
        <td>We should use spaces tooooooooooooo</td>
    </tr>
</table>

You need to set the table's width even in haunter's solution. Otherwise it doesn't work.
Also a new CSS3 feature that vsync suggested is: word-break:break-all;. This will break the words without spaces in them to multiple lines too. Just modify the code like this:

table.fixed { table-layout:fixed; width:90px; word-break:break-all;}

Final result

table {
  width: 100%;
  border: 1px solid #000;
}
th.from, th.date {
  width: 15%
}
th.subject {
  width: 70%; /* Not necessary, since only 70% width remains */
}

<table>
  <thead>
    <tr>
      <th class="from">From</th>
      <th class="subject">Subject</th>
      <th class="date">Date</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>[from]</td>
      <td>[subject]</td>
      <td>[date]</td>
    </tr>
  </tbody>
</table>

The best practice is to keep your HTML and CSS separate for less code duplication, and for separation of concerns (HTML for structure and semantics, and CSS for presentation).

Note that, for this to work in older versions of Internet Explorer, you may have to give your table a specific width (e.g., 900px). That browser has some problems rendering an element with percentage dimensions if its wrapper doesn't have exact dimensions.

See: http://www.html5-tutorials.org/tables/changing-column-width/

After the table tag, use the col element. you don't need a closing tag.

For example, if you had three columns:

<table>
  <colgroup>
    <col style="width:40%">
    <col style="width:30%">
    <col style="width:30%">
  </colgroup>  
  <tbody>
    ...
  </tbody>
</table>