I have recently gotten into a lot of Scheme programming but I am wondering which implementation I should use. Which implementations are considered the "major" ones and what are the various pros and cons of each? I'm running on an Ubuntu Linux box and right now I have PLT Scheme and Guile installed. Are there other implementations I should consider? Ultimately I would like to be able to create GUI applications, and any cross-platform support would be very nice. PLT Scheme seems to have a compiler, and if an implementation lets me create native binaries (for my Windows-using friends) then that's also a bonus.