Arachne is an object database for objects of the Deutsches Archäologisches Institut (DAI) and the Archäologisches Institut der Universität zu Köln. There's predominantly really old stuff - we're talking Antiquity here, so Greek and Roman things prevail.
Europeana is an art and object database that searches collections from various places in Europe. It's available in several different languages, and thus it's possibly worth it to try searching in more than one language for what you are looking for. Not so cool: There's a possibility to use some filters, but I haven't found a filter for the date yet, so you will get all kinds of things from all times.
REALonline is one of my favourite databases, one of the first places I search for stuff, and generally just wonderful. It is mainly a database for artwork, with some objects thrown in for good measure. The site is available in both German and English.
A last one just for the sake of completeness - archREAL is an object database with archaeological small finds; unfortunately this project sort of petered out, and has lain dormant since about 2004. If you want to use the database, you have to ask for a login (due to copyright reasons). It's only available in German.