About monetizing,besides donations I can suggest other things (some of which have been mentioned before):
- live support ( no really , this could come in handy to some people needing fast tips,the kind of people that don’t have the time to go to the wiki and forum )
- installing/configuring modx services ( ermm... though it’s already dead easy to install modx and have a site up and running fast ...)
- helping users choosing a web hosting plan. Many people would like to know a *good* web host that can cope with ModX’s needs.You could supply an affiliate link to a webhosting package that has decent amount of space ( 1Gb+) , bandwidth and , most important, features that make ModX run faster ( you said that it’s optimized for MySql 5.1+ and Php 5.x + - don’t remember the versions ) , and other features needed often ( apache server that has mod_rewrite on ) and has the basic security ( register_globals off). Actually, I propose you make a list with the best webhosts that have all of the above , and have affiliate links to them.I think there are people who would buy through them just to be able to make some contribution to ModX.
I think a banner or an unintrusive link about ModX hosting in the menu would prove VERY useful for both the ModX team and the users

.
- having a "download firefox" button/link.
https://www.google.com/adsense/ pays a certain sum of money for each persons that downloads firefox from their link ( 1$ for us downloaders, less for other countrys).
- You could also place adsense ads on the modxcms website , but that might degrade the user experience as some think ( though I would not agree , since contextual advertising is meant to be useful for the visitor - just depends on how intrusive they are ) .