TextDiagam is a web tool for drawing UML sequence diagram in pure text.

http://weidagang.github.com/text-diagram

Input:

object April Todd Monad April->April: Prepare food for lunch. space 5 note left of April: Lunch is ready. April->Todd: Todd, what are you doing? note right of Todd: @@ Todd->April: Well...\nI'm programming. space 5 April->Monad: How about you? Monad->April: I'm reading book. April->Monad: Good boy! note right of Monad: ^^

Output:

            +-------+                       +-------+   +-------+
            | April |                       | Todd  |   | Monad |
            +-------+                       +-------+   +-------+
                |                               |           |
                | Prepare food for lunch.       |           |
                |------------------------       |           |
                |                       |       |           |
                |<-----------------------       |           |
                |                               |           |
                |                               |           |
                |                               |           |
                |                               |           |
                |                               |           |