http://www.websequencediagrams.com/
Alice->Bob: Authentication Request
note right of Bob: Bob thinks about it
Bob->Alice: Authentication Response
Alice->Bob: Do something
Alice->John: Yo
note left of John: Hey
alt text1
A->B: text
else text2
A->B: text
end
A-->B: text
A->A: text
Just came across this little gem while trying to look for a cloud-based tool to create UML diagrams. Pretty darn cool!
You can create a Sequence diagram just by typing the relationships between the entities and it gets rendered automatically.
Check them out.