SourceMaking — is the best information source on the Web on such software development topics as design patterns, refactoring and UML. A lot of information freely available through the site's pages, so feel free to use bookmarklet to leave interesting chapters for further reading.
The first edition of this site was the outgrowth of a previous book project, Practical Relational Database Design, by Wayne Dick and Tom Jewett. The move online featured condensed discussions, an integrated view of database concepts and skills
Don't waste your afternoon drawing UML sequence diagrams. Just enter the description, and this online tool will draw it for you. It can emulate the look of many other popular tools.