I recently played through the three bonus missions of the original Splinter Cell that were added by official patch 1.3. Props to Ubisoft of old for giving away content for free. The Ubisoft of today would likely charge $5 per mission and require a permanent internet connection to play through them, despite this stuff being purely single-player.
Anyway, the missions are pretty decent for a freebie. They can be quite challenging, which makes them a good fit for Splinter Cell veterans. Plot wise, you end up going after the last remnants of the bad guys from the main game. This includes locating the hacker who was responsible for the IT attacks and infiltrating a stolen submarine.
There's about 2-3 hours of extra content here and it's pretty solid. It even features some new voice overs by the main cast, which is always appreciated. Good stuff.