Baseball Heroes: A series of insert sets, and later a stand-alone product, issued by Upper Deck.
In 1990, Upper Deck included a ten-card set of corporate spokes-jock Reggie Jackson, dubbed the "Baseball Heroes," in their high-series baseball packs. The cards were randomly seeded, with one card being found in about every sixth pack. What made it special is that Reggie himself personally signed 2,500 of those cards, and they were randomly seeded. They were extremely hard to pull since in the early 90s print runs of base brands like Upper Deck ran well into the tens of thousands of twenty-box cases. Caveat Emptor: all the Heroes cards include a facsimile autograph, so don't be fooled. The real autographed cards -- Jackson only signed the checklist -- look like the regular card, but have a special hologram. You'll know them when you see them.
In following years, Upper Deck continued the Heroes line with nine-card sets with sequential numbering and an un-numbered "header" card in each set. Reggie was #1-9, followed in 1991 by Nolan Ryan and Hank Aaron, in 1992 by Ted Williams in the low series and Johnny Bench and Joe Morgan together in the high series. 1993 it has Willie Mays in series one and current all-stars as "Future Heroes" in series two. Mickey Mantle joined in 1994's second series and Babe Ruth in 1995. Upper Deck then moved the set to the SP brand in 1996 with another set of Future Heroes, and Ken Griffey, Jr. in 1997 SP. The sets issued in the SP brands were decidedly tougher to pull (the '97 Griffey's were numbered to 2000 copies each) and more expensive.
Rumor has it that things fell apart when they tried to get Sandy Koufax for the '98 SP set. Negotiations broke down and they canceled. The bad blood continued as Sandy refused to appear in UD's excellent Century Legends set in 1999, although things were patched up in time for an appearance in 2000 UD Legends.