zenlists provides a handy JavaScript bookmarklet that allows you to quickly add a new item to any of your lists at anytime from your browser.
That's right - you don't even have to be within zenlists at the time :-)
To use this function simply drag this QuickAdd! link to your browser links bar.
Alternatively, if you are using Firefox, right-click on the link and select "Bookmark This Link..." and choose to create it in the "Bookmarks Toolbar" folder.