Some of the more popular tapas I've served included:
datiles con tocino (dates wrapped with bacon)
camarones rellenos (shrimp stuffed with pork)
gambas al ajillo (garlic shrimp)
albondigas (meatballs in saffron broth)
manchego cheese topped with membrillo and crisp serrano ham
croquettes with assorted aioli
patatas bravas (spicy potato sqeares)
Of course a plate of good olives, sausages (don't turn your nose at a good morcilla!) and preserved fish goes great nearby.
You mean tapas in the true sense (ie. Spanish nibblies) or the broader term for small portions of things?
I'll assume Spanish. Classic tapas include:
-tortilla (the egg and potato omelette)
-croquettes (cheese, ham, or plain bechamel)
-serrano ham, sliced thin, draped on a plate
-stuffed piquillo peppers
And of course X on baguette rounds. Some X ideas:
-marinated white anchovies
Google for more ideas, as there are *many* classic tapas, bread or no bread.
I don't know where you live, but you can start with a good farmer's market or upscale fancy-pants gourmet shop for the "obscure" stuff.