Proper Disposal of Objects in SharePoint
I just ran across this article on MSDN that is a fantastic read. If you do any work where you manipulate SharePoint objects via code, especially SPSite and SPWeb then you must read this. The article is outlines many of the gotcha's lurking in the object model that you may not be aware of. There is also another article that touches on object disposal as well but lists a few other areas to watch for.
If you are seeing performance, memory and stability issues in a SharePoint server that is running custom code then that code should be probably be reviewed while the above articles are fresh in mind.