Note that Potion can only be used because it got an official, not-just-on-a-printed-card, errata. A card like Energy Retrieval or Full Heal that has functional differences from their current prints CAN'T be used otherwise. (Incidentally, VERY few cards have errata like this. The only Standard-legal cards that have had this treatment are Potion, Rare Candy, Pokemon Catcher and Great Ball.) (There's a PDF on Erratas in the link Otaku provided that lists these, and a few text fixes to cards as well.)
http://pokegym.net/forums/view.php?pg=stdformat is a pretty good list of the older cards that are currently Standard-legal. They'll occasionally miss things or be slow to update it, though. (For instance, Base Set Caterpie and Weedle are Standard-legal. Unlike most of the Evolutions cards, these two were direct reprints, with everything from their HP to their attacks to their Retreat Costs the same. The Evolutions Surfing Pikachu, likewise, is also Standard-legal. Note that while Flying Pikachu and Electabuzz are almost identical to their original counterparts, their different resistances to the originals make them different cards, and thus the older ones are NOT Standard-legal.)
I should also point out that the Slowpoke that My Little Keldeo mentioned above ISN'T actually Standard-legal. The fact it can only get back Item cards is a functional difference, since Fossil Slowpoke can also get back Supporters and Stadiums due to them not being a separate class to Items at its time of printing.
I'll mention this since nobody else has explicitly stated it, but the ART doesn't have to remain the same. (Indeed, this is where most of the Pokemon reprints come from, alternate art versions of older cards.) Should be fairly obvious anyway, but probably worth mentioning all the same. ;p