Some of us need to create sequence diagrams while doing architecture design of product or a new feature. There are several online websites out there which can be used to draw sequence diagrams. Some of them are free & some of them are paid services. They support different features. Somewhere you can actually draw, somewhere you can type in text & some might support both. Personally I prefer to write text. That is quick & easy. We just need to type in the interactions between objects & tool will draw the sequence diagram for us. WebSequenceDiagrams is such a online tool. Its free version supports almost all the things that you would need to create a sequence diagram.