You can voluntarily pay whatever fee you would like. If you go to your settings page you can change the donation percentages. Personally I donate .25% of all my earning back to the pool .10 Pool Hosting, .10 Pool Management, .05 Stats dev. I personally feel that mining for free on a pool is stealing from the operators time and monetary resources which is why I voluntarily pay a small fee.
I agree, so I pay 1.2% voluntarily because I really appreciate the level of the experience on Eligius. And I just sent an extra BTC tip to the stats address because of how much work it must have been to deal with the recent issues. If you've been paying zero fees and you appreciate the level of support and service on the pool, now is the time to bump up your contribution.