I'm looking for something that allows me to work out an algorithm on a computer similar to how I would do it on a whiteboard. At work, I have a huge whiteboard that I can use for this, but at home, I don't (and can't). This makes it difficult to design algorithms when I'm working on hobby projects. I think better when I use drawings as opposed to text, so pseudocode is no good. I guess what I'm looking for is something like a flowchart program, but that allows a less rigid approach to designing algorithms.
I'd like the program to be cross-platform (Mac, Linux), but I'll be happy even if it just runs on Macs. Free is preferred, but reasonably priced programs are good too. I would prefer programs that people have used and had experiences with, rather than whatever turned up on Google, as I know of some flowchart and mind-mapping software, but haven't really been satisfied with either. I realise that a computer isn't the best platform for doing this kind of stuff, but assume for a moment that it's the only medium I possess.