Hi,
To create UML diagrams of design of the system either its Class, Sequence or Db design, I would always recommend Enterprise Architect (EA) by sparx systems.
http://www.sparxsystems.com/products/ea/index.html[
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It has all the features that a Tech designer needs to prepare detailed design. It also comes with an MDX installer which can be integrated with VS2010, so in case you are working with VS2010, its the best in the market.
The best thing is you can always sync up your code with design at any point of time so the design is always updated.
Besides EA, Visio is another tool which you can use for the design.
Hope this helps to make your decision...
with kudos,
Pradeep