Try to use the online version, someday, it Lite and saved my brain from bloating many times when I was absent without tools, if you need just one, I suggest this, but search for many available online tools
http://www.websequencediagrams.com/
LucidChart WebBased is also very fun (a free account with some restrictions)
http://www.lucidchart.com/
Desktop -
Eclipse UML2 Tools Is Perfect If u Need Desktop Version
Rise is Great And Freeware for C # Also Simple Sequence Charts
http://www.risetobloome.com/
* And * How sometimes people need UML to control thoughts and ideas of the mind, if U Do, since I see a U-point in Visio and Word, try to find some control of thinking (just a thought)
MindApp is the fastest and easiest, I really like
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