I've been using NClass for object/class modeling in class thus far.
However, I do not believe that it is capable of sequence diagrams, which is my next task.
I would prefer not to use Visio. Is there a good and free software solution for making sequence diagrams?
For me there are usually about 3 tools I prefer to use.
There is www.websequencediagrams.com that is quite cool. I'm having trouble taking it seriously, but it does a really good job at very low effort from my side.
Quick Sequence Diagram Editor might suit your needs. The syntax is strange, but easy to learn. The thing I like about it is that the diagrams are specified using text files, which makes me happy since I don't like the 'pure visual' approach used by the Visio and Rational tools.
Have a look at UMLet, a Free UML Tool for Fast UML Diagrams which is aimed at fast UML sketching.
Umbrello is a fairly comprehensive UML modelling tool. Perhaps unfortunately, it runs on Linux. However I have used it in a Windows environment by running in in a Linux Virtual Machine. Because the VM appears as a machine on your network (even if you do not have a physical network), it can share your file system and printer via network shares so you can use it fairly seamlessly. However all that may be more effort that it is worth, and you should consider other solutions.