It's time to back off a little from the letter X, which has been completely overused, especially in the last few years:

X-mas (there's actually an interesting story behind this variation)
Planet X (acceptable, since X is a variable)
The X-Files (the explanation of why the cases had this title was a little silly)
Mac OS X (somewhat acceptable, since X is the Roman numerical equivalent of 10)
Windows XP (Completely Unexeptable)
Sonic X
Sonic Adventure DX (Director's Cut?)
Megaman X
XXX (the movie, perv)
X2 (you already know how I feel about that title)
LXG
XBox (At least the Box part is true, kind of)
PSX (Playstation, Playstation 2, Playstation X?)
Static-X (pretty cool name, actually)
The XFL