Are expired patents still prior art?

As already stated by Steven and Edoardo, the short answer is yes for patents which expire or are abandoned after publication. It does not matter whether patents are still in force, as soon as they are published they are and remain prior art.

However, it is thus important to note that there is an important exception. Patents which expire or are abandoned before or without any publication will not be prior art. This might at first seem a scenario which is more an exception then a rule for most patent procedures, however it is especially relevant for procedures like US provisional patents which automatically expire after one year without publication.