Flowcharts: This information is not something everyone needs but if you are one of those who is involved in giving presentations, mind mapping or brain storming for personal or professional life, this (Flowchart) is a very useful tool. Being nothing but a visual representation, flowchart itself is now a separate entity when it comes to diagramming. Tools have emerged to deal with various kinds of flowchart representations and is a very competitive market.
Microsoft's Visio remains to be the king of diagramming tools available so far. And, it is as good as its price. And for preparing a couple of professional diagrams, you or me, don't need to own that. So, here are quite a few online alternatives, which will get the work done.
Flowcharts are nothing but a type of mindmaps. So, if you actually are looking for a solution to organize your thoughts, may be you need to try out the mindmap solutions first. So, why not give them a try with some solutions from the Brainstorming section of DiscComm.
Gliffy
Gliffy.com,simply put, is an online option for creating flowcharts. Yes, it is like Visio and No, it cannot open .vsd files. But Gliffy does create flowcharts very well for any of your needs, the only drawback is that you cannot save your work as visio-compatible files. Apart from that, this can be the solution for all your diagramming needs.
LovelyCharts 
Lovelycharts, at first glance, felt similar to bubbl.us at first but after using it a bit, found that it was pretty intuitive and much better than Visio too, when it comes to repetitive tasks. The drag and drop feature for creating child elements was so good, that creating a flowchart didn't feel difficult at all. And the best part was right clicking the object and changing the element type (ex: you can right click a process box and change it to decision box and vice-versa). So, you no longer need to worry about the object you select. Just complete your flowchart and worry about how the boxes look like later. Try it and then you will come to know how easy this is.
Project Draw (Used to be called Cumulate Draw)
Project Draw is pretty similar to Gliffy or shall I say Dia. It is not as highly polished as Visio but gets the work done.
Bubbl.us
Bubbl.us was already covered in the Brainstorming section but deserves a mention here for generating quick online flowcharts. It is completely based on flash. It is quick but the diagram keeps jumping here are there while you are working. Once you spend a little bit of time, it is pretty good. This is NOT an alternative to Visio but is a quick tool for generating/capturing ideas. Take a look and see if it suits your need.
And when we are here discussing about the online options, I think it would also be nice to have a list of the best 'desktop applications' for creating flowcharts a.k.a alternatives to MS Visio.