There are several tools available on the market that supports UML. For any project of size, it is recommended that some tool be used. Pencil and paper can suffice, but is difficult to maintain. These tools are listed here as a reference only with no particular recommendation. I personally use Rational Rose, but there are several excellent tools available. These are but a few of the tools I was able to find using the internet search engines.

Rational Rose 2000
Rational Software Corporation. This is the cadillac of UML tools. The three authors of UML all work for this company.
URL: www.rational.com

Together 4.2
TogetherSoft Corporation. Peter Coad who founded Object International Inc founded the company. He is the co-author of the Yourdan-Coad methodology.
URL: www.togethersoft.com

WithClass 2000
MicroGold Software Inc.
URL: www.microgold.com

Visual UML 2.6
Visual Object Modelers
URL: www.visualobjectmodelers.com

Visio 2000
Microsoft Corporation. Visio surprisingly is more than just a flowcharting tool
URL: www.microsoft.com/office/visio