Inspired by something read in the Humanspace Adventures playtest document, although I vaguely recall seeing similar in a blog comment somewhere, or possibly in the multi-layered morass of detail that is HackMaster...
Each time your character levels up pick one ability score.
Roll higher than the current score on a d20 to advance it by 1 point (max 18).
(note: this doesn't count as being during gameplay, so Jeff's Big Purple d30 Rule doesn't apply.)
Intent: this little gimmick models increasing capability as characters gain progressively more experience and versatility. The 'roll over' mechanic makes it easy to improve on a gimp ability score, but offers less chance of improving those lovingly polished Prime Requisites.
Yeah, it's a little bit Runequest for some tastes, and a shuffle away from the strict purity of "3d6, in order", but I'm an obsessive user of Ability Checks (LL, p55).
Randy Glasberg, syndicated cartoonist