Pretty much anything that a player has worn or touched has been included as a relic. Jerseys, bats, gloves, hats, pants, helmets, catching gear, balls, bases, walls... it's all been there. I recall stirrups being used in some Donruss products about 6 years ago (Absolute Memorabilia comes to mind). Has anyone used socks or underwear yet? I'm sure sweat bands have been used. And on the non-sports side, props have been cut up, the Berlin wall crushed up, and horse hairs cut up. Upper Deck made what look to be beautiful entomology cards for Goodwin Champions. I wouldn't know, I don't own one. And because plain swatches of stuff gets tired fast, companies have tried lots of non-relic relics. Manufactured patches, stamps, jewels, and, in 2003, coins.
As far as I can tell, there has been only one set ever issued containing actual money in the card. The 2003 Topps Gallery Currency Connection set. Well, two sets - the HOF edition contained coins as well. Twenty-nine different cards were issued between the two, each containing a different coin. The Gallery set contained coins from the player's country of origin - the Francisco Rodriguez card above contains a coin from Venezuela. The Hall of Fame version pairs players with coins dated from players' career years. For example, Lou Gehrig's card includes a Buffalo nickel minted in his last full year.
This is one of the few relic sets I want to collect. Unfortunately, the cards are hard to come by and are usually expensive. This is my second card from the regular set (I also have Andruw Jones). I have nothing from the HOF set. It's a process, of course.
Incidentally, Topps continued the experiment of coins in cards in another set that year - the 2003 Topps Tribute World Series Subway Series Fan Fare set (that's a long name, for sure) contained subway tokens to celebrate those years when the World Series was held only in New York. But since then, coins have stayed out of cardboard. At least in baseball.