You can create multiple user accounts.
Open Sytem Preferences > Users & Groups then select the Password tab.
Click the gold padlock icon bottom left corner of the pane to make changes. You may be prompted for your admin password.
Click + (plus) to create a new account.
Barry, what Silverlovely is talking about is a killer feature in Chrome where you can have profiles and switch between them instantly. These profiles don't share cookies, cache or anything which is awsome from a developer standpoint. From a user perspective, you can have different extensions for each profile to. Some people may want shopping specific addons and developers will have their set.
An elegant way for your browser to have multiple personalities depedning on what you want to do. Right now, Apple Safari sees the world with Rose colored glasss whereas the power users, want more. Hence why most people I know even with new Macs (non-techs alike because they listen to their tech friends) are running Google Chrome.
Add browser profiles, and I would dump Chrome for good. BTW, Safari 6 is WAY better that 5.x. When an extension crashes now, it doesn't cause all tabs to reload which cause me to jump ship like it were really on fire! Still, if a tab does crash, all the other tabs reload whereas Chrome does not.