There are a couple program types that use the TSK extension:
1. PhotoImpact Quick Impact
2. Skins (themes) for Pocket PCs or PDAs
3. Trend PC-Cillin File
This is also used for 'task' settings. If you can't find anything to un-zip it, try WinRAR. I am at work so my searching ablities are lessened, but I will search more when I get home.
You could say I've been around.