Every .NET developer who has come anywhere near SharePoint development has, at one time or another, seen, heard of, or used Jan Tielens' SmartPart. It's a great control and very effective for rapid application development within SharePoint - no need to learn web part coding, just whip up a quick user control and off you go.
We do a great deal of SharePoint user control development in our shop, for several reasons: 1) when you work on billable time, clients want the fastest, most effective solution to a problem not some esoteric argument on what is 'pure' SharePoint development and what is not, 2) It's easier to hire and get new SharePoint developers up to speed using .NET controls they are already familiar with, 3) it's far easier to turn over user controls to our clients for future code maintenance than try to give them a crash course in web part development, and 4) I don't know about you but I have absolutely no interest in hand-coding UI elements that I could otherwise drag-and-drop in the IDE (but then, I'm known to be a lazy programmer, so take that for what it's worth).
Because of this, we have developed our own web part wrapper for hosting user controls. Similar to the SmartPart, it's designed to be a low overhead control that can be deployed for rapid development scenarios but we needed a bit more flexibility than the SmartPart offers. We've been using it for quite a while and have had great success with it. We're releasing the source code to the community so everyone can do the same and also to see what kind of improvements and enhancements all you smart coders out there can come up with.
If you're looking for a way to host user controls in SharePoint and you would rather write and own the code yourself than use a third-party control, give it a try. Sean Skinner has posted the code over on his blog for you to download. Take it, make it your own, and let us know what kind of cool stuff you do with it.
Direct any questions to either Sean or myself via comments on these posts so everyone can participate in the discussion.