Online Russian dictionaries

Monolingual dictionaries
Мир словарей / World of Dictionaries

Includes dictionaries on a huge range of themes, including
- architecture - Архитектурный словарь
- business - Бизнес словарь
- history - Исторический словарь
- medicine - Медицинский словарь
- politics - Политический словарь
- religion - Религиозный словарь
- sociology - Социологический словарь
- philosophy - Философский словарь
- finance - Финансовый словарь
- economics - Экономический словарь
- law - Юридический словарь
- criminal slang - Словарь воровского жаргона
- Dal dictionary (acclaimed Russian dictionary) - Словарь Даля
- names - Словарь имён
- computer terminology - Словарь компьютерного жаргона
- measures and weights - Словарь мер и весов
- Russian surnames - Словарь русских фамилий
- thesaurus (synonyms) - Словарь синонимов

Many more sites to come

Bilingual dictionaries

Anonymous dictionary - nice and simple, not cluttered with advertising. Provides alternative translations and examples of uses and collocations, and provides both aspects of verbs. Inexplicably, though, it always seems to give the same definition twice and count it as two entries.

Rustran - seems reliable. Provides lots of alternative translations. Has a lot of product advertising.

Abby - downloadable dictionaries

Freedict - Russian-English and English-Russian. I wouldn't use this - I found it kept giving me translations for completely different words to the ones I entered.

Many more sites to come