I'm looking for a tool, the simpler the better, to create UML based activity/sequence diagrams. This is all I really need to do, so I don't need a full-fledged UML based application like Enterprise Architect or Visio. I'd like something very simple to use, and preferably open source, but that isn't a requirement.
Try JUDE community edition.
Here is a really simple online one:
Pretty good one, nice and simple, open source...works for both Java/C#, can generate code as well.
EDIT: Just realized you want activity/sequence not class...leaving NClass ref in case of value to someone else...
For non-class specific UML, http://www.umlet.com/
Are you using VS2010? This might be useful: http://visualstudiogallery.msdn.microsoft.com/en-us/8dba2cec-c3c4-4782-950f-8396036635fc
If you use Eclipse there is UML2Tools plugin.