QuickTime Player is free on Macs, and the free version will play and export in .mov (QuickTime format). You need the Pro version to export in other formats, like DV (Digital Video), the same format that many video cameras use and what you need to import into iMovie.
A full list of differences between the Player and the Pro version from Apple is here. And you can download the latest QuickTime Player at at Apple.
To figure out which you have and what version (as outlined in this Apple Knowledge Base Article) go to System Preferences and click on QuickTime, the blue “Q” icon. After QuickTime starts, select QuickTime Preferences from the “QuickTime Player” menu. In the Register tab, it should read “QuickTime 7 Pro” under “Registration Number.” If not, then you need to buy QuickTime Pro, and you can upgrade to QuickTime Pro at Apple.
If you need to work with Windows Media Player files with a .wmv extension, then try Flip4Mac. The free version will play wmv files in QuickTime, while the $29 Player Pro will import wmv files and convert them to .mov, and the $39 Studio version will export .wmv files.
Perian is also a possibility. Called the Swiss Army Knife for QuickTime, it’s a free download and it supports the AVI, DIVX, FLV, MKV, GVI, VP6, and VFW file formats.
Further complicating things is the version of QuickTime Player that is included in Snow Leopard the OS X upgrade that was released for sale last week; it’s called QuickTime X, and it’s capabilities are less than QuickTime Player 7. But, if you install Snow Leopard and you already have QuickTime Pro installed, Snow Leopard will keep it for you.
MacInTouch has a FAQ on Snow Leopard and QuickTime X, and a rundown on how to use QuickTime X alongside QuickTime Pro 7.