Part of the collection of the numismatic collection of the Staatliche Museen in Berlin is available online. The quality of the images is very high and one can enlarge both obverses and reverses. Detailed descriptions and notes also accompany the images. There are multiple ways to search the collection. Previously the interface was only in German, but Dr. Karsten Dahmen of the Münzkabinett has just recently informed me that they have launched an interface in English, which can be accessed by clicking "en" on the top right corner of the start page. To visit the website and begin learning about Berlin's wonderful collection click here.
(Image: Obverse of a sestertius of Titus depicting the Colosseum flanked by the Meta Sudans and the porticus of the Baths of Titus. Münzkabinett of the Staatliche Museen in Berlin, object number 18200446.)