The precise formula for XTOL is a Kodak secret; however, Kodak has filed patents and issued MSDSes on XTOL, so its rough composition is known. If you're interested, you can look up
US patent #5,853,964. Paul Lewis's
Mytol is very close to this, except that it lacks some of the preservatives, so it probably won't last as long.
As others have noted, Ryuji Suzuki and Pat Gainer have both published mix-it-yourself phenidone/ascorbic acid ("PC") developers with reputations for good keeping qualities. Although I've never used it, Suzuki's
DS-10 is often compared favorably to XTOL, although I've seen comments by Suzuki himself that DS-10 is a somewhat more specialized developer; it's intended as more of a fine-grain developer rather than a general-purpose developer. I've used Gainer's
PC-Glycol, which is a much simpler formula that's probably less similar to XTOL than DS-10, but it still works quite well, in my experience. PC-TEA is another popular Gainer creation, but I've not used it. Likewise, I've not used PC-Gly-TOL, which is a variant of PC-Glycol that's designed to come closer to Mytol (and hence XTOL).