What is a good cleaning agent for components

And for metal surfaces a perfect cleaner is neoblank -- used exceedingly spaingly, cleans the surface with no residue and no risk of marking -- this is a huge quantity (75cl) -- can be found in 50ml packs which will last you for ever
https://www.amazon.com/Neoblank-Stainless-Steel-Lubricant-Neodisher/dp/B01MYLK4EV

Never clean the face of a component. The letters (on/off, source, etc.) are usually silk screened and anything other than water will melt them off. This is why used equipment, found in audio stores, is usually dirty (they won't take a chance of damaging anything).

@dweller makes a good point about proceeding with caution. If you are comfortable that the lettering won't be removed, I learned about Endust for Electronics from Vlad Lamm. My line stage had gone in for updates some years ago, and it came back looking brand new. I asked Lamm what they did- that was the answer. Didn't leave any greasy residue. I spray it on a clean microfibre cloth and apply without overdoing, then use another microfiber cloth to wipe dry. Obviously, you don't want to get this near platter of turntable, tubes or the like, but for metal casings, does a wonderful job. 
Also, for plexi or acrylic, there is a spray cleaner that is safe for Apple computer screens, with a very soft fiber towel. I not only use that to clean my computer screens, but also to wipe carbon fiber shelving, or other things like display windows on gear. Also works well on high gloss ebony finish speaker cabinets- one towel for application, another for drying/polishing. Doesn't scratch. 

neoblank is <10% alcohol and 30% mineral oil -- used in minute quantities it is safe for lettering (I found it as Liebherr supplies it with all their fridges)

For perspex and other plastic surfaces a cleaner like Ultra Lite is ideal
https://www.g-collective.com/print-supplies/standard_products/ultra-lite-glass-plastic-cleaner-mini