Here’s some stuff I have found useful:

OAIster: A OA documents search engine.

ePrints-UK: A search of the UK’s academic OA document repositories.

Directory of Open Access Journals: An index of OA journals.

PubMed Central: NLM archive (free for articles >6 months old).

Obviously Google Scholar and Entrez deliver more results, but with Open Access, at least you know you aren’t going to get another “pay $$$!” message.