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.       |           |
                    |------------------------       |           |
                    |                       |       |           |
                    |<-----------------------       |           |
                    |                               |           |
                    |                               |           |
                    |                               |           |
                    |                               |           |
                    |                               |           |
------------------\ |                               |           |
| Lunch is ready. |-|                               |           |
|-----------------| |                               |           |
                    |                               |           |
                    | Todd, what are you doing?     |           |
                    |------------------------------>|           |
                    |                               | ------\   |
                    |                               |-| @_@ |   |
                    |                               | |-----|   |
                    |                               |           |
                    |                       Well... |           |
                    |              I'm programming. |           |
                    |<------------------------------|           |
                    |                               |           |
                    |                               |           |
                    |                               |           |
                    |                               |           |
                    |                               |           |
                    |                               |           |
                    | How about you?                |           |
                    |------------------------------------------>|
                    |                               |           |
                    |                         I'm reading book. |
                    |<------------------------------------------|
                    |                               |           |
                    | Good boy!                     |           |
                    |------------------------------------------>|
                    |                               |           | ------\
                    |                               |           |-| ^_^ |
                    |                               |           | |-----|
                    |                               |           |