wg-gesucht.de is actually one of the standard websites when you're looking for a shared apartment in Germany.
Generally speaking, the term you'd be searching for is WG (short for Wohngemeinschaft). This way, you'll find a few more like wg-suche.de, wohngemeinschaft.de, easywg.de etc.
You can also try a few general real estate websites like immobilienscout24.de or immonet.de -- they also provide the option to explicitly search for WGs.
That said, the housing situation in Heidelberg is rather difficult. In this city, the rental prices are among the highest in Germany.
It could also be worthwhile to check local newspapers. For Heidelberg, this would be the Rhein-Neckar-Zeitung. This newspaper is also availble online, and in the real estate section there is an option to search for a "WG-Zimmer" (room in a shared apartment).
It might be relevant to your question what you'll be doing in Heidelberg and how long you're planning to stay:
If you'll be a member of the University, check with the International Relations Office to see if they can provide any further assistance. You can also try contacting the international department of the Studentenwerk Heidelberg.
(In Germany, a Studentenwerk is responsible for providing additional services to students of a university, most notably housing, i.e. dorms.)
