Firefox only (Windows/Mac/Linux): Firefox extension Fast Dial replaces blank, empty tabs with thumbnails of your favourite sites. Fast Dial may look a lot like Speed Dial (and it is), but it also has several cool features of its own that set it apart. For example, you can set custom keyboard shortcuts for any of your Fast Dial thumbnails, create Fast Dial folders that you drill into for more thumbnail bookmarks, drag and drop items to reorder them, and customise the look and feel of every aspect of the Fast Dial page.
Fast Dial also integrates with your Firefox bookmarks, so adding a thumbnail to your Fast Dial page is as simple as bookmarking it and filing it in the Fast Dial folder. (That also means that if you're syncing bookmarks with Foxmarks, for example, Fast Dial favourites will sync, too.) Fast Dial is available from the Firefox Add-ons site, but they're a couple of versions behind, so make sure you get it from the link below.