I am currently using SiteGround - the original subject of this thread - for about 20 sites. I have been through about a dozen web hosts over the years. I am very happy with their service. And yep, I have MODx running nicely. They’re very on top of keeping their system running efficiently. I’ve seen them move my site between servers twice with no downtime or interruption in my workflow. From time to time someone will come along and abuse the shared hosting environment, but they are very fast at detecting and resolving this.
I won’t go into any more detail.. suffice it to say that as far as my needs go for a shared host, they’re everything I need and more.
Here’s my shameless affiliate link -
http://www.siteground.com/index.htm?referrerid=10263 . I wouldn’t include this link if I didn’t think it was of value to people because I take my fledgeling membership in the MODx community seriously.
Now, for anything that is mission-critical or even what I would call "business critical", or for any sites where you expect great deals of traffic (don’t ask me to quantify that - I’m sure you can decide what sites can live in a shared environment and what can’t) - I have also had a good experience with Dedicated Virtual servers from Media Temple.
Thanks everyone for the other suggestions here.