The VLC Media Player is an open source multimedia player for various audio and video formats, as well as streaming video and DVDs. Free to download, it works well with both Macs and PCs. The media player includes a Snapshot feature (under the Video menu) which allows you to capture stills from video. Just pause at the suitable frame and take the snapshot. These snapshots may then be used in PowerPoint, Keynote, or OIV presentations.
For more information about the VideoLAN project (including the VLC Media Player), click here.