Even though we also recommend it for dinner, Mesón Las Conchas (Rua Mayor 16) has the best tapas in Salamanca. You can eat at the bar or at one of the tables next to the bar (the ones without tablecloths). The pork kebab tapa, the "huevos rotos" with chorizo, and the "farinato" (a chorizo-based local favourite) are delicious (Wine + tapa 2.10€, draft beer + tapa 1.80€).
You can combine it with Las Caballerizas, a bar located in one of the old stables next to the Faculty of Philosophy and which for this reason is very lively during the week (though it stays open on weekends too). The small Serrano ham sandwiches and Spanish potato omelettes, free with a drink order, are very good (wine + tapa 1.50€). It's on Calle Tostado (the street heading down from Palacio de Anaya, next to the Cathedral), through the first entrance on the left.