Snagit requires Microsoft Windows 2000, XP, XP x64, Vista 32-bit, or Vista 64-bit. It may be possible to use wine to run Snagit but I have not tested it.
If you want to make screenshots in Linux, simply use the command kconsole. If you want to record desktop videos, you may have a look at screekast [1] or istanbul [2].
Another possibility is to use a Windows machine to connect via vnc to your Linux Desktop. On the Windows machine you may use Snagit for recording desktops or making snapshots.
[1]
http://sourceforge.net/projects/screenkast
[2]
http://live.gnome.org/Istanbul