Pre-Rookie Cards
Rookie card is a confusing and unfortunate term, since many of the cards we call rookie cards are not from the players' rookie seasons. Non-collectors often email me to say "you say 19xx Topps is so-and-so's rookie card, but he was a rookie in 19yy." And so I try to explain.
Adding to the confusion is the idea that a card is a rookie card only if it was printed by a major card company. Some players' first cards were part of minor or regional issues, and these are not considered their rookie cards. Because of their limited distribution, these pre-rookie cards are usually harder to find than the "official" rookie cards. Below are the pre-rookie cards of some well-known players, along with their commonly recognized rookie cards.
1961 Lake to Lake Packers
1961 Lake to Lake Packers football cards were distributed (I assume packaged with dairy products) by the Lake to Lake Dairy in Wisconsin. Half of the cards in the set are plentiful, and the other half were severely short-printed and are extremely difficult to find. I estimate that the non-short prints outnumber the short prints ten-to-one.
The set includes cards of four hall-of-famers whose "official" rookie cards were not printed for at least two more years. Of those four cards, only Herb Adderley is easily found. The other three are short prints.
Ray Nitschke
Ray Nitschke started his career in 1958, but he didn't appear on a card from a major company until 1963. Here are Ray's 1961 Lake to Lake card and his 1963 Topps rookie card:

Willie Wood
The official rookie card of Willie Wood is also a 1963 Topps:

Willie Davis
The official rookie card of Willie Davis is a 1964 Philadelphia card. He also appears on a 1960 Kahn's Weiners card as a Cleveland Brown.

Herb Adderley
Herb Adderley's rookie card is also a 1964 Philadelphia card. When they finally gave Herb a card, they misspelled his last name. And they misspelled it the next three years, too!

1962 Post Cereal
1962 Post Cereal football cards were distributed on cereal boxes and had to be cut from the boxes by hand. There are 200 cards in the set, so you had to eat a lot of cereal to collect them all. The set includes four hall-of-famers whose official rookie cards were issued in later years.Bob Lilly
Bob Lilly's official rookie card was issued a year later, in the 1963 Topps set.

Larry Wilson
Larry Wilson's official rookie card was also in the 1963 Topps set.

Gene Hickerson
Gene Hickerson's rookie card was a 1966 Philadelphia card. He had another pre-rookie card in the 1960 Kahn's Weiners set.

Jim Johnson
Jim Johnson's rookie card appeared in the 1964 Philadelphia set. Jim gave us some funny poses.

1959 Bell Brand Rams Sid Gillman
1959 Bell Brand Rams football cards were distributed in bags of Bell Brand potato chips and corn chips. They are beautiful cards, with sturdy cardboard stock and a glossy finish unlike other issues of the time. Unfortunately most of them were cut off-center, many badly. It is a challenge to find the cards at all, much less nicely centered ones.
Hall-of-famer Sid Gillman appeared in the 1959 Bell Brand set as the Rams' head coach. The next year he moved to the Los Angeles Chargers of the newly-formed American Football League. Gillman's official rookie card is in the 1960 Fleer set. I am still trying to find a PSA 9 of this card.

1961 National City Bank Browns Len Dawson
Hall-of-famer Len Dawson appeared on a 1961 National City Bank Browns card two years before his 1963 Fleer rookie card. Dawson had also played for the Pittsburgh Steelers.

1961 Nu-Card
1961 Nu-Card football cards featured star college players, many who later went on to the NFL. While there aren't any hall-of-famers in the set, a few of the players are notable.
Ernie Davis
Ernie Davis was a Heisman Trophy winner who was drafted by the Browns and would have teamed with Jim Brown had he not contracted leukemia. Here are his 1961 Nu-Card pre-rookie card and his 1962 Topps rookie card:

Roman Gabriel
Roman Gabriel was drafted by both the Rams and Raiders in 1962, and he opted for the Rams. His rookie card is also a 1962 Topps. (This is one of several 1962 Topps cards that picture the wrong player in the inset photo.)

John Hadl
John Hadl was the starting quarterback for the San Diego Chargers for eleven years, until Dan Fouts took over. Hadl then went on to spend five more years with the Rams, Packers, and Oilers. His rookie card is a 1964 Topps.

1972 Sunoco Stamps
1972 Sunoco Stamps were given away at Sunoco and DX service stations with purchases of gas. The entire set is huge--624 stamps--so it is not surprising that it contains a few pre-rookie cards (or rather, stamps). Five of the pre-rookie cards are Hall of Fame players.
There is an update set, as well--82 more stamps!--and the update set includes a Jim Langer pre-rookie card. I will post that here when I get one.
Jack Ham
Jack Ham's Sunoco stamp has a dark image. His rookie card is a 1973 Topps card, and it's much more attractive.

Jack Youngblood
Jack Youngblood's Sunoco stamp also shows him in action. I'm not fond of the action shots on these stamps. The images are so small that you can't tell who the players are. Youngblood's rookie card is also a 1973 Topps.

Art Shell
Art Shell's Sunoco stamp shows him with his helmet on, but at least you can tell it's him. His rookie card is a 1973 Topps.

Dan Dierdorf
Dan Dierdorf's Sunoco stamp is a nice head-and-shoulders shot. His rookie card appears in the 1973 Topps set.

Mel Blount
Mel Blount's rookie card is a 1975 Topps, which I don't have yet. Here is his Sunoco stamp.