Reader Josh writes in with the full list of Gmail search operators and says they work when creating filters, too.
By default Google ANDs all data that you put into the filter fields. To use an OR condition all you have to do is put || between two strings (you can also just use OR). For instance to have a filter work that marks messages from josh@email.com and joshua@email.com I'd just put: josh@email.com || joshua@gmail.com in the From field.
There are tons of available operators, like minus (-) (or NOT matching this criteria.) For example, I have all email addressed to tips at lifehacker.com skip the inbox and labelled "tips" - unless the message is from Adam or Keith or one of the bossmen. Handy.