As Nick Wilde (hey, my name is also Nick!) already said, Xmarks is a great solution to sync your bookmarks.
However, I feel the need to give some additional information.
The Xmarks extension syncs your bookmarks (and passwords) in various browsers.
It does this by uploading the bookmarks to their website, and then the extension will download them to the other browsers you're using, provided you've installed the extension, and you're logged in to the extension with your Xmarks account. Syncing passwords is also available.
With the premium subscription you can also sync on mobile devices.
This extension is designed for Internet Explorer, Mozilla Firefox, Google Chrome and Apple Safari (Only for Mac OS X). There is no official version for Opera.
However, with a few 'hacks' you can install the Chrome version in Opera. You will need at least version 16 of Opera for this, which uses a new engine similar to Google Chrome's (if not the same one).
EDIT: There are plans to make an official opera extension, but until it's actually available, you'll have to use the method below.
What is on your roadmap?
We plan on continuing to expand where you can access your data. This includes more browsers (Opera) and more mobile devices (we just added Android and BlackBerry, etc).
- Go to "opera://flags" and search for "Bookmarks". Enable the bookmarks toolbar if it isn't already enabled by default.
- Afterwards go to "opera://settings" and select "Show the bookmarks bar" to display the toolbar (if it isn't already selected).
- Install this extension: Download Chrome Extension. It will allow you to install extensions from the Chrome Web Store.
- Go to download.xmarks.com, and click on the button with the Google Chrome logo on it. Then click on the red "+ Free" button.
- You might see a notification underneath the bookmarks bar saying that the extension was disabled. Click on the "Go" button to go to the extensions page and click on "Install" next to "Xmarks Bookmarks Sync".
- Click on the "Options" button that appears.
- Follow the setup wizard ("Run Setup Wizard").
For all other browsers you can just go to download.xmarks.com and click on the "Download Xmarks" button for the current browser. After installing, run the setup wizard. (I have also included direct links to download the extensions at the top of my answer)