RegisterClientScriptBlock
I saw this recent MSDN TV episode about ASP.NET client side script handling. As a sworn hater of client side scripts I played the clip in horror, but found that as long as I can assign these scrips to my pages serverside (and dynamically) they are actually pretty cool.
The first use I got for this snippet was for dynamically assigning CSS templates to pages based on user preferences:
In PreRender (or Init):
this.RegisterClientScriptBlock("css", "<link href='"+ myVariableWithCSSFilename +"' rel=stylesheet/>");