DLL: How do I avoid 'Permission Denied' when re-compiling?

Use this batch file to release IIS' lock on your DLL: 
 

net stop iisadmin /y  net start w3svc

 
The disclaimer of course, if you're worried about stopping your web site temporarily: don't develop DLLs on production machines, and if you need to deploy a new version, use a different ProgID. 
 
You may also find the need to use KILL.EXE, if iisadmin refuses to be shut down. KILL.EXE is found in the Windows Server 2003 Resource Kit. As long as the .EXE is in your system's path, you can modify the above script as follows: 
 

net stop iisadmin /y  kill iisadmin  net start w3svc

 
(You may also need to start other required services such as FTP and/or SMTP.) 
 
In Windows 2000 and above, you can use a much faster process by issuing the following command (again, at a command prompt or in a batch file): 
 

iisreset

 
You can also issue stop and start commands independently: 
 

iisreset /stop  iisreset /start

 
If your object is hosted in MTS (or an application running in its own memory space), you should be able to just unload that application / package. Similarly, for objects hosted in COM+, shutting down the application from Component Services should unlock any holds IIS has on your DLL. 
 
If you are developing your ASP files using Visual InterDev, then IntelliSense may be doing you a disservice. Since this feature actually hooks into your custom COM objects (once you've created them using their ProgID in a createobject() statement), this places a lock on the DLL similar to the one IIS places on it. So if you are editing an ASP file which calls the DLL in question, you can recompile without rebooting by simply closing the ASP file.