I've got Hirens (who doesn't?), and I keep a copy of Knoppix around...
But in the main I stay in WinPE/RescuePE mini-win . I can scratch-build my own, but I have had pretty good luck with WinBuilder builds. The software takes a lot of the pain out of it.
I keep a miniXP and a mini7 - Both are very basic, all that's onboard is nic drivers, my private Paragon Partition Manager, some serious file managers, keyfinders, and off-line registry tools. That means I don't have to rebuild them very much, and I haven't touched either one in years. YAY, because images suck.
Apps are nearly infinite - Anything that runs portable will probably run (see PortableApps, PortableFreeware), with the odd dll needing to be added to the image... But I don't load apps into the image. I keep them on a separate thumb, and keep that updated, which is far easier to do, and has become a fully automated system. All I have to do is plug the thumb into my server at night, and AV defs, installs, and portables are all upgraded to the live version sitting in the server. All of that (update) is my own software, and man, was it worth doing.
And, if my mini finds that thumb on boot, The Start Menu is populated with the thumb's apps, mydocs and etc are redirected to folders on the thumb, and my cmd environment/path and toybox are automatically set... system32 gets a full load of commands, a dll directory is copied into Windows, a directory of .reg files is automatically processed, and an inf directory is pathed to allow for way more capability.
With the thumb, my RescuePE becomes a very sophisticated platform.
Look into MiniWins. They're a gas.