I use Opera on my Mac and my Windows machines. I think you can add plugins to FireFox and IE to achieve the same functionality.
I bookmark sites that I want to keep a list of. I organized them into folders. I also enter details into the bookmark's notes/description area, and this allows me to search for those terms, and locate any bookmark quickly.
Sites that I want to check out (but not bookmark) are opened into tabs. I have Opera set up to remember my open tabs. So, each time I open Opera, (even after reboots), I have a set of sites waiting for when I have time. When I am done with them, I simply close the tabs, and they are gone.
For sites that lie between bookmarks and temporary tabs, I use a feature in Opera called Speed Dial. It is like a visual grid of nine bookmarks, and allows me to keep URLs handy, but not occupying the tab bar real estate.
The new version of Opera (9.5, currently in beta) has synchronization features tied to a free Opera account. So, my bookmarks and speed dial sites are synced between my Windows and Mac computers without any fuss. It works really well for me.
I hope I didn't get too technical with the above. Feel free to ask any questions, if clarification is needed.
Jim