I'm wondering which app might be the "best" for compacting the registry. I've used all of the following: NTREGOPT, Auslogics Registry Defrag, Registry Mechanic and Registry Defrag Free. Is one of these better than another -- or are they all about the same?

I like Auslogics Registry Defrag. I'm not sure if one is better than the other as they are all supposed to do the same thing right?

I sort of thought so, but some seem to be faster than others.

Well, NTREGOPT is a good and safe choice, but for example Free Registry Defrag has Analize feature, which is good to have, so you dont defrag, when theres actually no need to.

What you can do, is to Analize with FRD, and then defrag with NTREGOPT, if there's need to. If you do prefer NTREGOPT.

Auslogics Reg Defrag has Analyze feature too Here, Last Freeware Auslogics Reg Defrag.

I use Auslogics.

I'd think they all pretty much do the exact same thing.

I use FRD because it has the Analyze Registry feature, and it's still freeware.

Is there any general rule of thumb as to how frequently -- or at what level of fragmentation -- this should be performed?

I'll perform compacting registry each month if your machine runs noticeably slower after a few months of work.

FRD recommends not registry defragging if the fragmentation is at 2% or under since Windows is typically always at that level and will quickly jump right back into that 2% zone in no time. I usually only use FRD if I notice it at 5% or higher, but really you don't need to use such programs but maybe once per year or at the most twice per year.

Do note that Windows (XP at least) does something all on its own and can reduce the size of the registry unexpectedly and very rarely, and it's much better at it than any of the reg defrag tools are ever possible of achieving.