Adsense is probably the simplest, but you need a lot of targeted traffic to earn money with it.
Asking your members to buy from your Amazon and other Affiliate links, since you get 4% or more commission depending on how many people you can refer to Amazon and your other affiliates like Host Gator, Fat Cow hosting, etc. This can be difficult since people have to spend money at your affiliates which is not easy since lots of people are cheap and don't like spending cash on stuff online.
Selling Ebooks and digital products like scripts, software can be a good way to earn money, but it is not easy.
Providing a very useful service like Posting Packages like Forum Promotions and ask for donations.