After I started playing SR as a player again, I also got the urge to do a little GMing. I'm a busy man, so a big, continuous campaign doesn't suit me, so I'll rather want to run a few semi-linked one-shots with more or less the same characters. For this, I need suitable adventures.

What I need is scenarios, which can be run in (preferably) a single night (5-6 hours of gaming), or at most two. SR4 would be prepared, but SR2 or SR3 would also be completely okay. Are there such things available for purchase or free download?

Check out the Denver missions, they are free on the main shadowrun site. Should play in about 4 hours but with all the goofing off and sidebars it could be stretched to 5 or 6.

http://www.shadowrun4.com/missions/downloads/

You can go to the archives linked off that site for the first two "seasons" you can convert those to 4th or play them in 3rd. You can get the Manhattan Missions on drivethrurpg.com you have to pay for those though.

Woah, that's a treasure trove right there, thanks a bunch. How linked are they? Can they be run separately, and will someone jumping in during, say adventure 10 feel left out and not have a clue what is going on?

Paying is not a problem, by the way, if I am paying for quality.

The Denver Missions have a lot of repeat NPCs and some references to earlier adventures in the series, but other than that aren't really connected. There actually are different dialogs to read if the PCs did an earlier adventure or not. There is no big plot so you can jump in whenever. The other seasons I don't know I never ran them I just know they are there.

If you do decide to buy the Manhattan Missions they are less than $4 a piece so it's not super expensive.

I have GMed the first 4-5 Denver Missions. If the Manhattan Missions are anything like them, I wouldn't pay for them. Preparing the Missions is almost as much work as creating them from scratch.
Just think up somehting a corp wants to have done in secret and go from there.