A very interesting service for creating Sequence Diagrams: Web Sequence Diagrams.

Sample diagram created with only a few lines of text:

AdapterService->ContentService: ImportArticle
ContentService->Essex: AnalyseArticle
Essex->ContentService: ArticleAnaysed
ContentService->ContentService: HitArticleAvailable
note right of ContentService: FollowUris
ContentService->SnapshotService: HitArticleAvailable

The diagram above was created with the following lines of text:

AdapterService->ContentService: ImportArticle
ContentService->Essex: AnalyseArticle
Essex->ContentService: ArticleAnaysed
ContentService->ContentService: HitArticleAvailable
note right of ContentService: FollowUris
ContentService->SnapshotService: HitArticleAvailable