Not sequence diagrams but a tool to make diagrams in general yEd - http://yworks.com http://yworks.com -- been using it since a long time - it's Java based, so

Not sequence diagrams but a tool to make diagrams in general yEd - http://yworks.com http://yworks.com -- been using it since a long time - it's Java based, so works on all platforms and it's free to use.