Mac OS X 10.9 Mavericks
As it's a free upgrade that gives you more apps, improved features across the board, and better performance, there is really no reason not to get Mavericks on your Mac.
Microsoft Windows 8.1 ( Spanish)
If you're a dedicated Windows 8 hater, the update to Windows 8.1 isn't going to change your mind. For everyone else, this collection of tweaks, fixes, and new features is useful, but everything here should have shipped in the original version last year.
Microsoft Windows 8 Pro ( Upgrade, Latin America, Spanish)
Microsoft makes an aggressive, forward-thinking, and bold statement for the future of PCs with Windows 8, and vast security and speed improvements more than justify the $40 upgrade price.
Apple Mac OS X 10.6.3 Snow Leopard
Intel Mac users will like Snow Leopard's smartly designed interface enhancements, and its Exchange support is a must-have (especially with Outlook for Mac on the way). With a ton of technological improvements, Snow Leopard is worth the $29 upgrade fee.
Mac OS X 10.5 Leopard
The grace of Leopard's interface enhancements makes productivity more pleasurable with a Mac, as more than 300 functional and fun features top off this update.
Apple Mac OS X 10.4.6 Tiger
Though its new user features have mixed appeal, Tiger's technological changes and speed improvements make this an enticing upgrade, especially for those who passed on Panther.
Windows Vista Home Basic
Windows Vista Home Basic is essentially warmed-over Windows XP, Windows XP SP3. If you're currently happy with Windows XP SP2, we see no compelling reason to upgrade. On the other hand, if you need a new computer right now, Windows Vista is stable enough for everyday use.
Apple Mac OS X 10.3 Panther
Panther adds new and useful features, but at the expense of some old standbys. Current Jaguar users need not apply.
Red Hat Linux 8.0 Professional
Red Hat remains a solid Linux contender, but Windows switchers or dual-booters should stick with SuSE for easier installation.
Mac OS X 10.2 Jaguar
Take a good, hard look at Mac OS X 10.2 if you're adding a Mac to a Windows network--Jaguar's new tools can't be beat. But home users, beware the $129 upgrade price if you're not looking for networking options.