Mountain Lion's version of Safari has a couple of neat tricks up its virtual sleeve. My favorite of these is called Tab View, which'll let you see all the tabs you've got open and swipe between them. Here's what it looks like:
You can invoke it in a number of different ways; for example, you can click on the "Show all tabs" button that'll appear on the toolbar when you have multiple tabs open.
You can also press Shift-Command-\ (View > Show All Tabs).
The way I like the best, though, is to just pinch with two fingers on your trackpad. Nifty! (If this isn't working for you, make sure you have System Preferences > Trackpad > Scroll & Zoom > Zoom in or out turned on.)
Once you've got Tab View open with whatever method you've chosen, use a two-finger swipe to move between existing tabs. If you prefer, you can instead use your left and right arrow keys, or click the small dots at the bottom of the window to jump around.
When you find the tab you're looking for, click on it to pull it to the front (or pinch out on your trackpad). That's all there is to it!
And heck, just because I'm feeling generous today (and by "generous," I mean "super caffeinated"), here's a tip that's not 10.8-specific. If you only want to move to the next tab without getting into Tab View, there's a keyboard shortcut for that: Control-Tab. To move to the previous tab, the shortcut is Shift-Control-Tab.
I confess that as much as I like Tab View, I use those shortcuts almost as often. After all, my browsing habits generally involve only a few tabs at a time. For those of you who simply must keep 45,874 tabs open, though, this new feature is probably gonna save you lots of frustration.