Try Use Online Versions Sometime , It's Lite And Saved My Brain From Blow-up Many Times When I Was Out With No Tools , If U Need Simple One I Suggest This But Search Out Many Online Tools Available
http://www.websequencediagrams.com/
LucidChart WebBased Is Also Really Awesome ( Free Account With Some Limitations )
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 Diagrams
http://www.risetobloome.com/
*And*As Sometimes People Need UML For Managing Mind Thoughts And Ideas If U Do , As I See U Point At Visio And Word , Try Find Some Mind Management ( Just Thought )
MindApp Is Fastest And Easiest , I Like It So Much