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Apple won't release OS X 10.9 Mavericks until fall, but you can get many of the best features announced in the next version of the desktop operating system through third party apps and plug-ins.
Finder Tabs and Tags
You could've gotten tabs and tags in the Finder for years if you just switched Finders. Third-party app Pathfinder provides both of those features and more. As a file browser for power users, you can split windows, open up a terminal window, queue file copies, put folders on top in file lists, and a whole lot more. If you want some of the features Apple will likely add in future versions of their desktop OS, you can have them now with Path Finder.
If you prefer to stick with the traditional Finder but want to add features, get TotalFinder instead. It'll add tabs, and you get add tagging separately with an app called TagIt. While neither option will provide the highly integrated user experience you'll get with OS X Mavericks, it should be enough to tide you over through the summer.
Multiple Display Enhancements
Apple thankfully fixed many annoying multiple display quirks introduced in OS X Lion when they announced Mavericks. Unfortunately, no third party app can put two full screen apps side by side. You can, however, improve your situation with a somewhat pricey app called Multimon ($10). It sticks a second (or third) menubar on your other monitors, makes it easy to move windows between each display and resizes them appropriately, and provides customizable hotkeys for quick access to its functions. If you only want multiple menubars, check out the free Secondbar instead.
iCloud Keychain brings password syncing to all your Apple devices, but it'll remember credit card numbers, form information, and Wi-Fi passwords as well. The downside? You can't sync with non-Apple devices. Fortunately, better solutions exist now. Lastpass does a great job of managing your passwords, but if you want those "eWallet" features you'll want to sign up for Dashlane instead.
Guess what? You've had synchronized notifications for a long time and you didn't even know it. Or maybe you did, if you were using Growl. Long before Notification Center, Growl handled notifications for many apps in exactly the same way but also worked with other apps to co your notifications across devices. It'll take a bit of setup, and it won't work as magically as it will in OS X Mavericks and iOS 7, but you you can do it. Check out our guide for Windows or Mac for more information.
Calendars, Maps, and iBooks
Calendars lost its leather look, and Apple added Maps and iBooks to the crew of included apps. If you want the simpler, leather-less look of Calendars, just download Lion Tweaks and turn it off. To get maps you can send directly to your iPhone, just use Google Maps and push them with an app like myPhoneDesktop or Send to iPhone. Lastly, you don't really need iBooks when you already have Amazon Kindle. While that won't help you if you have a bunch of iBooks content already purchased, Kindle users can access their books on pretty much any device. If you don't want to lock yourself into the Apple platform, Amazon at least lets you choose your devices.
Of course, OS X Mavericks packs in a bit more than you'll find in this list but third-party apps can't replicate all of the features (including the secret ones we have yet to discover). Let these tools tide your over for now, and get the real thing in the fall when OS X 10.9 officially launches.