We get about 40-70 support requests every day. Downloads fail, payments went wrong, files are corrupt or people report fraudulent software. About a year ago we installed OTRS - the open ticket response system. The software package is PERL based and Open Source. It runs smoothly on every newer Linux web server and if you have a lot of requests to answer or if you want to outsource your support team: This is the best solution I have seen so far.
You can download the package from the OTRS web site, but you need some webmaster skills to work with it properly! It supports POP3 mail pick up, so you can create your extra support email address and OTRS picks up the mail and puts it nicely in a queue. Follow-ups are then sorted into a tree and the whole process of closing tickets or escalating them is very slick!
I am still trying to get the maximum answer time down to 12 hours and we are getting closer to that. Yet it can take up to 48 hours still, until we got our new staff members trained!