Opera Unite has come under fire from some leading security experts. In upcoming Opera 10 the Unite feature actually puts a web server inside the browser. According to Opera's CEO this will make it easier for people to host their own websites and to share files over the internet.
It is interesting that this is now being touted as a great thing; the ability to host a website from a PC was something that Microsoft was criticized about with Windows. The www publishing and hosting service was viewed (and turned out to be) a large security hole.
Now when Opera does it, it is great tool that will help millions. While Opera's foot print may be small and will help it stay off of the radar of many malicious coders it is really only a matter of time before someone sees this as a challenge and dives in.
Opera Unite is not yet in final form but there is a publicly available alpha version that you can play with