XBill is probably the least-likely of these games to be forgotten, but I would be remiss if I covered old Linux games without mentioning it. XBill has you protecting a series of computers against an army of 'Bill' clones who seek to install a virus disguised as an operating system. You do this, of course, by clicking on each Bill to cause them to explode. Naturally.
Alas, disregarding the thinly-disguised jab at Microsoft, XBill is only an okay time-waster without a whole lot of staying power. In addition to clicking on Bills, your other tasks consist of using a water bucket to prevent the virus from spreading over the network and re-installing operating systems in machines that were shut down but not infected. The complexity never increases, only the number of Bills that you need to deal with, and the arrangement of computers to defend. This ends up as a rather slow difficulty ramp, with about ten very easy levels before the challenge starts to appear. However, since the only fail state is getting all computers infected, you will probably find yourself getting bored of clicking on Bill before you actually lose. I also noticed that the game logic seems to get sluggish with a large number of Bills, even on a modern PC, which seems to result in a few unfortunate misclicks at that difficulty.
XBill is written by Brian Welligton and Matias Duarte in 1994, initially for Motif on AIX, then migrated to Athena widgets and ported to a whole host of other operating systems (including Windows). Eventually, in 2001, a GTK front-end was added.
The following is a video playing the first 25 levels of XBill. After this, I let Bill win.
XBill is still distributed in most distributions, and from its official web site. Other platform versions can be found there too.
More general information on old X games, including build tips can be found in the Old X Games article.
If the above video does not work for you, it is also available on my YouTube channel.