A short list of dictionaries well-known in Islamic Studies, that are now digitally available online. There are of course a lot more dictionaries out there on the web, but I have included here only those resources which scholars would be inclined to trust and use, and are freely accessible.
Please leave other websites in the comments – I will update this post if there are some good suggestions.
baheth.info – gateway to Classical Arabic-to-Arabic dictionaries.
ejtaal.net – a bundle of currently used dictionaries (note: unclear whether copyright has been honored.)
Online Dictionary of Arabic Philosophical Terms – conglomerate of scholarly glossaries.