I spend a fair amount of time working on interactive web applications and I would love to be able to standardize on using a single browser. FireFox would be the obvious choice because it is fairly standards compliant and available for most platforms.
I saw an article on Slashdot this morning on Mozilla SunBird: a standalone calendar application that can use any WebDAV server to support a shared event space for multiple users. I just spent a couple of days creating a web based multi-user calendar from scratch for a custom project - it would have been good to use something already written, but it was also nice having total integration with our business data models, etc.
In any case, there are many great open source projects that enable consultants like me to put custom systems together quickly and inexpensively. I just started offering a 1/3 discount for any work supporting open source projects - I am booked solid work-wise for several months, but when I do have time available, I hope that customers take me up on this discount offer - I really do believe that individual consultants and small (perhaps even medium) sized companies win BIG with open source.