By far the most popular and widely know flavour of the GNU/Linux distribution's. Our tutor was 'encouraging' us to try Linux so we 'encouraged' him to give us a run through making a Windows / Ubuntu dual boot set up. I offered up my Laptop for sacrifice, which turned out rather good, everything just worked, wifi the lot.
Since then there have been a couple or more iterations so, college course finished, I subsequently upgraded to the 10.04 LTS version.
The laptop works so well I intend to hold it at 10.04 over the life cycle of the distro.
I got my daughter a used, original Acer one net-book (Linpus Lite and 150Gb HD) - after a few weeks the damn thing crashed and went black screen. Good old Google offered up a recovery process, which I'd link to if I could remember it! Much to my surprise it worked first time. (your mileage may vary) This left us with a bootable machine and no OS. The first version of Ubuntu Netbook Remix (UNR) had become available so in it went. Another positive outcome.
I also have another Acer one, it came with Windows XP, I made it dual boot Ubuntu 9.04 (full version). I have never had any problems with this machine so have not upgraded it beyond 9.04, I do however use a variety of USB boot alternatives. A very handy tool. With the coming of 10.10 support for 9.04 has ended, therefore I have added another partition and installed 10.10 netbook edition. It is very good (or so I thought at the time).
August 2011: We had two netbooks running Ubuntu with the Unity desktop, as above. I'll try anything once (within reason) and I wasn't about to take the critisim of Unity as read, however I wanted to tweak one to be a temporary router and finding the terminal never happened. I decided to hunt again, Mint 11 was out so that cured one of them. For the other I had a go with Fuduntu www.fuduntu.org