Hi,
Yes, revive adserver is a free openx adserver. they do not provide any plugins. This is a good adserver but comes with only little feature so, if you want to work an additional things like (making Sign up / Login / Advertiser / Publisher / feature adding to your website) you need addtional plugins to get these features.
Here, I suggest reviveadservermod they provide Plugins for Revive Adserver to boost your Revive Adserver performance.
Thanks!